Rolled, if possible
If you are going to let them sit on top of the beams in the garage, rolling them will keep creases down to a minimum. A J-24 captain that I used to crew for used to roll his sails at the end of each race, as did the whole fleet, so I am going to assume this is best for the sail. Pretty easy to do, also. As for storing outside, even subzero weather should not hurt them too much, in comparison to what the sun does to 'em. It may make the fibers contract a bit, but if the sail is clean, I don't think that should not be a cause for concern.Rob