You should NOT store your car on Jack Stands (unless it’s #4 below)! Doing it this way has the shocks/suspension fully extended and that's not good. I have 4 good options for you to consider:
1. fill tires to max tire pressure and keep weight on the cars suspension
2. if your still worried about your tires, roll the car to help prevent flat spots every few week
3. if 2 isn't good enough, buy a set (4) tires cradles pads.... something like this
http://www.griotsgarage.com/product/...FcG77Qod0T_m0Q
4. put jack stands under your suspension (this can be tricky and dangerous if not done correctly) to keep the car's weight on the suspension.
Porsche recommends #1. I do #1 and #3.
Mark