I'm not sure about cash game b/c of size of pot, recklessness of opponent, etc., but I do know that in a tournament it is protfitable to push all in if need be with KK no matter what because the odds of you having KK when another player has AA is so low that is a insta push or insta call all in. Plus, if its your KK vs Ax, you are still 60-70% depending on dead cards, draws, etc. In a tourney, I am never laying down KK preflop, but I agree with sometimes not reraising preflop, as your opponenet never expects it. I know its been done to me before and I'm usually shocked to see it turned over if it goes to a post-flop all in.

I mucked KK preflop once in a brick and mortar cash game and felt like an assclown cause I let outside factors get involved (don't ask its way too embarassing)