I think that there’s some consensus that tires should be bagged in some way to slow the evaporation of softening agents. Starting this past winter I “bagged” my tires in standard 33gal garbage bags. Yesterday, however, I bought some 55gal garbage bags from Home Depot and found that a single bag will go over a stack of 4 tires perfectly. Much more convenient than 4 separate bags.
So my best tires are now bagged. If I had a set of fresh rain tires, they’d be bagged too. But since my rain tires are 3yrs old, whatever was going to evaporate out of them, probably has.