Some feats assume a creature can take multiple 5-ft. steps even when the feat doesn't allow the creature to
A creature really can't take more than one 5-ft. step in a round unless a special ability explicitly allows it. The Player’s Handbook on Take 5-Foot Step says
You can move 5 feet in any round when you don’t perform any other kind of movement. Taking this 5-foot step never provokes an attack of opportunity. You can't take more than one 5-foot step in a round, and you can't take a 5-foot step in the same round when you move any distance.
You can take a 5-foot step before, during, or after your other actions in the round…. (144)
(Emphasis and double emphasis mine.) That's in a round, not just during the creature's turn. Since a creature's relative round begins at the beginning of its turn this absolute round and ends on the same initiative count on the next absolute round, a typical creature that either takes a 5-ft. step or moves at least 10 ft. on its turn really is prohibited from taking another 5-ft. step during its round unless it's explicitly allowed to do. (Also, to be clear, as a creature's usually finished its actions on its turn, a creature can't usually take a 5-ft. step off-turn.)
Authors frequently omit this explicit exception. You've already listed the Setting Sun maneuver shifting defense [stance] (ToB 73) and the feat Evasive Reflexes (ToB 30) but similarly affected are feats like Cunning Evasion (PH2 78), Net and Trident (CW 114), Opportunistic Tactician (Dragon #340 87), and Orien Battle Stride (Dra 143). (Note that there's no consistency here—it's not like a lone author or editor is responsible for all this confusion; instead it's a general malaise that hovers on the game's edges, awaiting discovery and sending those who do into fits of seething anger. Also note that I may be projecting.)
This means that—despite, for example, the benefit of Evasive Reflexes or the description of shifting defense strongly implying otherwise—, the rules do, in fact, still prevent creatures employing such effects from taking multiple 5-ft. steps in a round unless those creatures have another special ability explicitly allowing multiple 5-ft. steps. Obviously…
This is terrible!
…And it's made even worse because sometimes a feat will follow the rules. For example, the feats Psionic Sidestep (Expanding Your Mind Web column "New Psychic Warrior Feats, Powers, and Ranged Weapon Enhancements") et al., Pursue (ECS 58), and Sidestep (MH 28) recognize, accommodate, or work around the 5-ft. step rule, making it look like those other feats should be limited when, in all likelihood—especially given the rarity of other special abilities that allow a creature to take more than one 5-ft. step in a round—those feats shouldn't be so limited.
Urge the DM to institute a house rule!
This DM strongly urges a house rule saying that any feat that seemingly intends to allow multiple 5-ft. steps in a round (like those listed above) instead allows the creature to move up to 5 ft. without provoking attacks of opportunity; this is not an action. Such a house rule both avoids conflicts with actually taking a 5-ft. step and soothes the seething anger of folks who think too much about this stuff.
About that Tome of Battle Q & A
Fectin mentioned in a comment on KRyan's fine answer an answer from the Sage that addresses the feat Evasive Reflexes. Edited for readability but not content, here's that exchange:
Questions: The Tome of Battle includes a feat named Evasive Reflexes. This feat is raising a lot of questions on the message boards. Here are the main questions that I hope you can shed some light on:
- If I have both Evasive Reflexes and Combat Reflexes, and I am normally entitled to four attacks of opportunity per round, what is the maximum number of 5-foot-steps per round that I can make using Evasive Reflexes?
- Do the 5-foot-step(s) that I make using Evasive Reflexes count against the total number of attacks of opportunity that I can make in a single round?
Thanks in advance.
- Four. You can take a five foot step each time an enemy provokes.
- Yes they do! Essentially you are replacing the attack with the move, but it still uses up an attack of opportunity!
This comes from a thread of compiled Tome of Battle wisdom purportedly from the Sage that's archived on various sites around the Web. (Most accessible is likely this Oct. 2015 thread on EN World, but there's also a Mar. 2015 archive and an undated one on different Web sites.) That purportedly isn't intended cast any doubt on anyone's honesty; instead, the questions and answers—while in the style of later Ask Wizards and Sage Advice Online columns—appear to be just a collection of largely unsourced Customer Service email responses, which were, in the day, notoriously unreliable and contradictory.
This answer is not immune. The benefit of the feat Evasive Reflexes says
When an opponent gives you a chance to make an attack of opportunity, you can instead immediately take a 5-foot step. (ToB 30)
Seriously. That's the whole thing. So rather than make an attack of opportunity, a creature opts to take a 5-ft. step. The benefit of Evasive Reflexes does not say the creature spends an attack of opportunity to make that 5-ft. step. In fact, considering the benefit's weird gives you a chance language, an argument can be made that even a creature that's exhausted all its attacks of opportunity can take the 5-ft. step granted by the feat's benefit!
So while the answer could, were it properly sourced, relay designer intent, the answer doesn't address what the text says, and the answer isn't official errata (also see here). The answer could make for a relatively reasonable house rule… at least until the player of a feat-starved scout starts eyeing Evasive Reflexes so his character can skirmish better without the burden of Combat Reflexes. Then such a player should try to persuade the DM to reconsider the sage's ruling.
On the stance shifting defense, the same thread includes this exchange:
Question: Does the shifting defense stance grant you a 5-foot step before the attack roll is made, making it miss because of your movement, or after the attack roll is made and consequently results in a failure?
Answer: The shifting defense maneuver grants you a 5-foot step after you know that the attack misses, because the stance states that it is activated upon a failed attack.
That's because the description of shifting defense says
You duck and move as you dodge your opponent’s attacks. Slowly but surely, each attack gives you the opportunity to move across the battlefield.
Your ability to read your opponents' moves and use their strength against them allows you to shift your position during a battle. Each failed attack gives you the split-second you need to move without drawing attacks.
While you are in this stance, you can make an immediate 5-foot step each time an opponent attacks you. Moving in this manner consumes one of your attacks of opportunity in the currrent round. You cannot move in this manner if you have no attacks of opportunity remaining. This movement does not provoke attacks of opportunity. (ToB 73)
Emphasis mine. So that Q & A is rules-accurate, but, seriously, saying failed in the second paragraph but not in the first and third? I don't even. Anyway, a defender using shifting defense in conjunction with the feat Robilar's Gambit (PH2 82) means the attacker provokes 1 attack of opportunity for making an attack and, if the attack misses, the defender can opt to spend 1 more attack of opportunity to take a 5-ft. step (or, if you prefer—I know I do—move up to 5 ft. without provoking attacks of opportunity; this is not an action.)