Say my ratfolk unchained rogue has all the time in the day to hide his dagger before going out on adventure. He meticulously positions it just right inside of his wrist sheath, trying to make sure it is as well hidden as possible. Let's say he has the better part of an hour to get it right.
Can I take 20 on the Sleight of Hand check versus any possible future perception rolls?
And the reason for that, is because there are consequences of failure.
Retry? Yes, but after an initial failure, a second Sleight of Hand attempt against the same target (or while you are being watched by the same observer who noticed your previous attempt) increases the DC for the task by 10.
If you fail a Sleight of Hand check, you are discovered. So you cannot possibly know all people that will look at you or will search your body for that hidden knife. That means you will try to hide the item on your body at least twenty times, rolling from 1 (terribly hidden) to 20 (masterfully hidden). But how can you know how well you hid it without a feedback?
Since taking 20 assumes that your character will fail many times before succeeding, your character would automatically incur any penalties for failure before he or she could complete the task (hence why it is generally not allowed with skills that carry such penalties). Common “take 20” skills include Disable Device (when used to open locks), Escape Artist, and Perception (when attempting to find traps.
Taking 10, on the other hand, means you did try to hide the item using your best knowledge and made sure that it will be well hidden against almost anyone who tries to search for it, assuming you are skilled enough and trust your abilities (has ranks on it). But that does not count Luck, represented in the game system by the dice roll. So, on average, your dagger is very well hidden by taking 10.
In other words, Taking 20 assumes you will fail a few times.
Another point to take into consideration is that fact that you do not make a single Sleight of Hand check and oppose whoever attempts to find your concealed dagger, but one check per attempt.
If we look at Disguise, we have the following text:
You get only one Disguise check per use of the skill, even if several people are making Perception checks against it.
On Linguistics we also have something similar aswell:
The Linguistics check is made secretly, so that you’re not sure how good your forgery is. As with Disguise, you don’t make a check until someone examines the work.
Both skills mention that you only make a single check.
Sleight of Hand lacks anything like that on the skill description, it simply mentions it is an opposed check. Notice how Stealth also does not mention anything like that, so everytime someone rolls a Perception check to notice someone hidden, the characters must both check against each other (an opposed check), though they could say they are Taking-10 depending on the situation.
While it's possible to take 20 on almost any roll there are certain possible situations where using sleight of hand to hide a dagger sheath will and will not work. However a lot of this is down to the approach by individual GMs, some are tolerant and others are not, your outcome may vary.
There are endless variations of situations where preparing in advance can be helpful and endless more where it is of no advantage, so to answer the general question 'Can I take 20 on a skill check versus any possible future opposed rolls' then no, taking 20 cannot be used against any possible future situations.
Straight from the SRD:
Taking 20
When you have plenty of time (generally 2 minutes for a skill that can normally be checked in 1 round, one full-round action, or one standard action), you are faced with no threats or distractions, and the skill being attempted carries no penalties for failure, you can take 20. In other words, eventually you will get a 20 on 1d20 if you roll enough times. Instead of rolling 1d20 for the skill check, just calculate your result as if you had rolled a 20.
Now then, in my experience, some DMs do not allow this or will in certain instance will not allow it, but from strictly a rules stand point, yes you can.
