I have a rogue in my party who likes to constantly use stealth in combat after he makes an attack, in order to get the CA required for the sneak attack dice. He justifies this by standing far away and making a stealth check, and with his stealth skill ranks, this means a practical gauranteed stealth. Is this valid or is he making a mistake?
You and your player may be overlooking some things about how Stealth and hiding work. You should both read The Rules of Hidden Club, which explains both comprehensively. It's an excellent guide, and the official rulebooks don't explain it nearly as well.
Bear in mind the Stealth rules in the first Player's Handbook are broken and incomplete. They're repaired and amended in the Player's Handbook 2 on page 222. You can read the updated rules in the Player's Handbook update PDF from the D&D 4e Updates and Errata page.
If your Rogue player wants to hide, merely standing far away is not good enough. They have to meet several other conditions, too. Quoting Hidden Club:
In addition, your Rogue player may incur a penalty to their Stealth check for the move action they're using to become hidden. Quoting PHB2 p222's Opposed Check entry in the Stealth skill:
Once you're Hidden, it's not as simple as just staying that way. All your Rogue's enemies still know where they were before he became hidden, and he still has to follow the Rules of Hidden Club (as described in that page I linked) in order to stay hidden. For example, he has to maintain at least cover or concealment from any enemy, or he is no longer hidden from that enemy (who can then immediately inform the others).
So: get reading!
I can't find the link right now, but I believe Mike Mearls did say that the intention is for Rogues to be able to use Sneak Attack during most or all of their turns, so don't be nervous if that turns out to be the case.
Whether or not you're correctly using the (IMO convoluted and extremely difficult to adjudicate) Stealth rules correctly, I can assure you that a carefully-run Rogue can play absolutely by-the-book and remain stealthed about 95% of the time.
I was a stickler for rules adherence in a 4E campaign that got to 15th level over more than 100 sessions, and the Rogue player was a master at choosing exactly the right combination of Powers to keep himself out of sight. The result was a fantastically acrobatic ninja who was amazing fun to watch and fight.