Actually, it isn't the best thing for tires to sit for LONG periods of time on the same 'spot'(on the tire), flat or not. However, if they're out of the sun/weather,sitting on a dry surface, bearing little weight, and were in good shape to start with, it shouldn't be TOO big of a problem. I have a full-sized EE pipe cart, which I almost NEVER have used-I go out, lift it up one side at a time, rotate the tires about 1/4 turn, every so often--easy!
Margo