I've seen it mentioned sometimes, but I am not at all convinced that it's a concern.
The typical chicken feed--that is supposed to be appropriate at all temperatures--has a fair bit of corn in it already (I'm referring to almost any bag of chick starter, layer feed, etc.)
Of course, any feed will produce heat when it's being digested. I don't know if you can keep chickens cooler by having them digest most of their food at the cooler times of the day (morning, night). That might call for early-morning feedings, and evening feedings. Or maybe it would be better for them to nibble little bits regularly (digesting smaller amounts at once, thus generating less heat at a time.) Leaving food constantly available, so the chickens can decide when to eat, might make the most sense (usually also easier for the human, rather than having to feed at exact times.)
