nah, you want to get all your chips in all the time KK, it's basically the mortal nuts before the flop, the effective stacks are 30bb here so you are not near deep enough to flat here.  KK against an ace is a great spot here, 70% of the time you win, and are crushing the field.  The ace hits sometimes and that's too bad, but if you flat, and no ace comes, he will fold, if it hits he will push.  It's pretty easy for him.  Just be happy you got it in as a big favorite.

The math is if you are all in, the pot is about 25k if you are both all in preflop.  You win 71% of the time. You on average win 17,750 chips.  Making your stack on average, 23,750.

Now say you flat- 3,400 is in the pot, no ace hits, and he doesn't pair his other card, ~68% of the time

He c-bets like half the time maybe for half the pot, so add about 900 to the pot, 4,300.  You shove, he folds, so your stack is 20,100 (18,000-1,200)+ 4,300  

He hits the ace, bets, you fold, and this is considering you are folding everytime he bets his ace, when it's probably better to float at least one street, but we can cancel out this with the times you flop a set and he hits an ace.

After this your stack is 16,800, this is 16% of the time.

He hits his other card, and you get it in.

You win ~80% making your stack 31,000

You lose ~20% making your stack 6,000

So your average stack here is 26,000

20,100x .68=13,668

16,800x .16= 2,688

26,000x .16= 4,160

=20,536

So on average you make around 3,200 more chips by shoving than flatting, which is significantly better.  Haha so I hope that helped a little, I really was just going to give a quick, always get it in pre, but then I got caught up in why we want to get it in pre and it just kinda spiraled on from there.  Math may not be perfect, but I think it's close enough.