If you have them sitting on their treads, rotate them every month, or you'll get flatspots in the tread and steel belt that wont go away. I had severe flatspots in an extra tire I had after only about 3/4 of a year.
If you leave them on their sides, I've heard that their sidewall can deform and the tire won't work properly. I do not know peronsally about this, but I'd put them on their treads, cover them, and rotate them every month or two.
I wouldn't let them sit for more than 5 years. Tires and brakes are two things that nobody shoud ever skimp on for saftey reasons.