Inflate to sidewall; move car occasionally. I've parked vehicles OUTSIDE, IN THE SUN for at least 6 months at a time and found no such flat spots. Flat spots can be a problem, but the easiest way to avoid them is to raise the tire pressure. Since you won't be driving the vehicle, inflating them maybe even 20 PSI above sidewall will be just fine to ensure that you don't get any flat spots.
Inflating to sidewall will cure your acne too.
Inflate to sidewall; move car occasionally. I've parked vehicles OUTSIDE, IN THE SUN for at least 6 months at a time and found no such flat spots. Flat spots can be a problem, but the easiest way to avoid them is to raise the tire pressure. Since you won't be driving the vehicle, inflating them maybe even 20 PSI above sidewall will be just fine to ensure that you don't get any flat spots.