Personally I keep mine it’s going to be 9 degrees.When I hunt in the winter these rear heaters have been a life saver more than once. Second when these want to overheat I turn all heaters on to cool down the engine.
And at the opposite end of this scale…if in the summer you need/want to turn the heater on to assist with cooling the engine, also having the rear heater will assist in cooling the engine …along with the main heater.