My experience with bias tire flat spotting is the flat spot goes away after driving a few blocks. I've never had to have one shaved because of flat spotting due to storage.
I've had a few sets of bias ply tires that were permanently flat spotted from sitting for ages with low air pressure and full weight of the car. Essentially ruined.
If you're not driving your car for months it's beneficial to get the tires off the ground, or move it forward and backward every few weeks.
