I store my tires and bike's with tires off the concrete, not so much to prevent flat spots but to elude the temperature variance. Here the temps are well below freezing and I've always been advised to store them off the concrete, I presume because it maintains a cold spot that freezes the tires more easily near the contact point. Cars I store for the weather get moved every couple weeks so to not sit on the same tire patch all winter. Spares and unmounted tires I rack horizontally and off the ground as well. Here flat spots can become a real issue when sub zero temps are common and can be persistent, but my wing is rarely parked more than a couple weeks, so it's easy to park on the center stand and 2x4. My Virago is on its center stand and there front is jacked up just off the floor.
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Tom, in Mountain Home, Idaho
2002 GL1800 (Illusion Red) Non-ABS, 128k miles
Retired Air Force
"Audentes Fortuna Juvat"