One thing I don't understand is why everyone always says "it's probably not much of a mistake at 100NL or lower", refering to going allin preflop with KK.. while it's probably the opposite. Lower stakes are infested with passive players, who will only go allin preflop with AA/KK. So if you have KK, you'll always be screwed.

Odds of you getting KK: 1/221 (0.45%)
Odds of someone else having AA when you have KK at a 6max table: 2.45% (combined: 1/9020)
Odds of someone else having AA when you have KK at full ring table: 4.5% (combined: 1/4911)

(sure feels a lot more in practice tbh, sites "rigging" for action? :P)

So if you're always stacking off 100BB instead of folding when the signs are obvious, you're potentially dumping:
6max: -100BB/9000= -1.1BB/100= -0.55ptBB/100
full ring: -100BB/4900= -2BB/100 = -1ptBB/100

So yeah, increase your winrate by 0.5ptBB/100 or 1ptBB/100 (depending on SH or FR) by folding KK if the signs are obvious!