Since storage is always a problem - a problem that doesn't get easier to solve when you quilt - I "store" extra sheet sets for guest bedrooms on the beds. It's a bit of a pain when company comes, because I have to strip the 'extra' off, but otherwise it works.
I think you could also store extra sets of sheets between the mattress and box spring. Then who cares how they're folded? Or even if they're folded?