as a rule, I think that if you can still get xp from the dungeon, you should go there. Once that xp stops, stop going there.
my level 10 character still gets xp in the lower levels of the crypts, sometimes all levels if the spawns are good, so i see no reason why he shouldn't be able to take advantage of that xp. He is making progress according to his rest message, so again.
But isn't that not what this is about? This is about running into a dungeon in stealth or invis, taking all the goods, and leaving without fighting. I have a stealther, and its not above him to do this. It can be strategic. you don't KNOW for sure if someone is going to spot you, and you still have to take precautions with your moving (remember! when you move, you suffer penalties that you don't suffer when you stand still.) in fact, isn't there bonus xp for rogues who stealth past successfully? Or... did I misread that?
What I think is terrible, is when you're working hard to clear a dungeon, and you have a sneak taking all your loot before you have a chance to, WHILE you're there. More importantly, without RP. There are a lot of ways to rp a sneak right, and just following people around because they can't see you without offering interaction is cheesy.
I always make sure to freak my prey out.