Don't heat it unless you want to work there. As others noted, heated garages increase the rust (salt isn't corrosive if frozen in ice), it will also attract rodents. I'd check w/ Tire Rack about the storage temps - IIRC there are some tires that will be damaged by freezing, even if off the car (I know I've heard this for racing tires, but thought there was also s/t about this for some performance street tires). A couple of 2x2 sheets of plexiglass and you've got a keen new set of matching end tables
.