As I understand why this is important, covering your tires reduces (but does not eliminate) the Ultraviolet rays in sunlight that penetrate the rubber and cause the evaporation of the solvents near the surface that keeps rubber flexible over time.
As solvents near the surface leave, and the tires are not rotated at speed long enough to warm up (allowing solvents from deeper in the rubber to cycle to the surface), the surface will become hardened and crack (called checking).
When on the sidewall this can be seen as cracks or checkerboard lines in the sidewall. This process can occur between the treads (where is is harder to see) and cause deep fissures that can fail when the tire is finally rotated to speed.
Age matters even when you take proper care of your tires. NOT taking care of your tires reduces this life, it does not extend it. Like your batteries, the tires are designed to have a useful life when made. What you do with them can reduce that life; not increase it.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW