My vote is vertical, like the tire shops. If there is a flat spot, it will be gone shortly after driving. Horizontal, especially if you stack, causes the beads to close closer together and take a "set" that way, after a while. Then, when trying to mount on the rims, there is a problem with getting the beads touching the rim, creating problems inflating. Although may not be an issue on short aspect ratio, low profile performance tires. I had this problem some years back after stacking horizontal, and my tire shop almost couldnt get them to seal and wanted to sell me new tires. There are probably some tricks to help.
For instance, on big truck and tractor tires, my brother used to put lighter fluid, or something, on the bead as the tire and rim laid flat,with the hose attached to the valve. Then he threw a match on it, and "whooofff" the bead blew up against the rim, so the air could start sealing. Or something like that. The lighter fluid burned off quickly creating no problems. Pretty dangerous, so please dont try!! First time I saw him do that was in a wash bay of a gas station he ran. Scared me to death, but there was a water hose and buckets of water nearby.
To edit, after seeing the original post again, it probably wont matter in tires switched twice a year. I also see that they are already mounted, so horizontally is better, as you dont have to worry about them rolling around. My problems were with tires (only) stored longer.
Maybe just disregard my whole post................I need more coffee!!!
[ March 30, 2005, 12:17 PM: Message edited by: MAJA ]