If you're talking a season or two I don't think it matters much as long as you keep them out of sunlight. For longer periods I would guess that storing tires unfolded might be a good idea. As for longevity, there was that crazy mechanic for the Discovery team who kept stacks of tubulars for years and years in his cellar before using them. Apparently tires can improve with age like wine, although I can't imagine why it would be so