I would never fold KK prelop at 50NL or below. The chance is just too great your oppenent sucks ass and has TT or AJs. Ive folded KK twice preflop, both times in 3 way reraised pots where a short stack was obviously going all in anyway, and a tight, solid player was showing huge amounts of agression. Both folds were correct. It's so much easier to fold when you get to see the showdown.

Conversely, at higher stakes if you hold AA and a tight, solid player is showing alot of agression preflop, concider a smooth call -> c/r on the flop. This will not allow them to get away from their hand so easily, where as they might have been able to fold it preflop with only a few bb commited.