I have no idea where this wives tale came from. Aluminum is less prone to cracking and warping than cast iron is. It cools down faster, throws heat faster and is repairable, where cast iron has none of these features. 

Almost every car produced these days has aluminum heads, and some are completely aluminum, and how many do you hear about having cracking or warping problems? None. Cast iron on the other hand is very prone to cracking, especially in high heat conditions. I’ll show you a whole pile of junk, cracked cast iron heads out back, and there isn’t a single aluminum head in that pile, and we work with more aluminum heads than cast iron (by like 5 to 1).