If it does it isn't much. Occasionally I will forget to put the developer back into the heater and it gets cool in the tray and I process anyway and it doesn't seem to make any difference. I think my heating the developer becomes part superstition after awhile. My developer gets really old because when I heat it I am usually careful to pour it right back in the bottle and cap it so it doesn't evaporate between prints. Then when my volume starts to get low I add fresh developer to the old.