Is it okay to store bike tires in unheated garage over winter?
Very cold and very hot are generally not good. Zinn has written an excellent article on this topic, and notes that Michelin recommends storing in 5C to 35C temperatures. 

You should hang the tires, or keep them mounted and inflated. In general you want to avoid UV and ozone (so keep the tire away from a heater, if you put them inside), and store the tires in a cool, dark, well ventilated, dry place. 

I'd say just throw the tires in the garage, even though 0 F is below the recommendation of Michelin; I've left bike tires on bicycles outside in the winter at those temperatures, and the tires are fine. Of course, there may be some life cycle reduction, but the risks are tiny overall.