My understanding is that tires that are 'mounted' and aired up will not deteriorate at the same rate as 'unmounted' tires. That is why tires on the 'shelf' so to speak need to be sold and mounted within 2 years. Our costco won't even sell you a tire that is older than 2 years according to the code on the tire. My daily driver is a 2000 chev, still have the original tires on it, only 80k mileage though . . .
If your tires are mounted, and your car is stored inside a garage, I wouldn't worry too much about detoriation. Maybe take some of the weight off them (jackstands) in the off season to reduce flat spotting them.