I have always stored my bows by hanging them from a peg vertically, some have been hanging like that for years, a few 10 years or more. The other day I decided to shoot some of the bows I hadn't touched and found the limbs were out of tiller. I suspect the slight pressure on the bottom limb plus being stored in a hot out building caused the change.
I decided horizontal storage would be a better idea so I put in a bunch of pegs over my tillering tree to hold my "extra" bows.
