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If you can periodically roll the bike back and forth a bit it's not an issue. I'd worry more about leaving the tires sitting on a concrete surface, put some wood, rubber, carpet scraps, etc. under them if thats the case.
does your car get flat spots in the tires if you don't drive it for a month? maybe if you still have bias ply tires like the super swampers on my land cruiser. even if the bike tires did get flat spots it's a quick burnout to being round again.
I've experienced flat spots on large, heavy motorhomes that were parked for extended periods of time, especially in the cold weather. However, after running 10-20 miles they warmed up enough to return to their original shape.