I can easily see going back to hell being Dean's greatest fear at this point...no, second greatest fear. Sam getting his evil on is still his greatest fear in my mind. But going back would not be a happy thing. Yet I don't remember the crossroads demon saying anything about him being in hell for all eternity, so in my way of thinking the contract's been paid. He went to hell, he's suffered - Casteil got him out with the big get out of jail card, he shouldn't have to worry about it.

Now I could be incorrect in what the deal was, but seems to me Dean really shouldn't have to worry about this. No matter how they play it so long as I get to watch Sam and Dean I'll be happy. They'll do a top notch story no matter what and that's what makes this show so dang cool!