Doesn't even matter if you do get below zero. You do not need to add heat, unless we are talking about something to keep the water unfrozen. The chickens are more than capable of handling the cold. By adding heat, you are not allowing your birds to acclimate to the colder temps. You are actually putting them at risk, if you were to lose power to the heater, or the heater justs craps out, your birds would truly suffer then, and that would be on YOU.
Sorry, but I like giving them a little heat. Turns on at 4am turns off at 8am. Takes a bit of the chill out of the air in the barn. If the power goes out, we have a generator, if a light bulb burns out I have extra.