If your seat heaters are hot enough to harm seat covers, you would not be sitting on them. My wifes car has seat heaters, they will let you know when to turn them off because you butt gets hot,but it wouldn't melt seatcovers. If they would harm seatcovers, it would harm you leather upolstery.
When you are up to your butt in alligators, it is hard to remember your intent was to drain the swamp. Stay focused!