If that was a reason for head cracking then you should never fly in rain because it cools down the front cylinders to the extent it causes head cracking.
Ah, yes, but...
Even if you're flying along in air at -40 then the front 2 cylinders are being provided with energy from the combustion within. The point about an extended period at idle is that the whole cylinder will cool, then be suddenly subjected to a lot of energy from one side (the inside).
Still, you might be right, urban (and rural) myths abound.
An interesting thread drift...
TOO