Michelin site says to keep upright, not laid flat and stacked one on the other.
If they are on the car well that part is solved. I did buy new tires last winter and store them upright awaiting installation.
Personally I check the air pressure to keep as recommended on the plaque.
Bought a big, and heavy, mat, 3/4" thick, that farmers use for their cows and cut out four approx. 2' X 2' squares that I drive up on to when hibernating. So the tires are not on the concrete and there is some "give" to the rubber mat. Check the tire pressure a couple of times during the winter. Never had a problem, three winters, and don't really anticipate any as the storage facility, my basement, is heated.