it really depends on circumstances- if you are in a humid climate & your fabric becomes damp with humidity- and is left on something that is (acid-y) it may discolor- or mildew- but under normal circumstances it takes a long time for any effect- it is more important if storing a quilt- that is an heirloom that is put away for an extended period of time to pay attention to how you store it- your fabric that will be moved around- (aired) on a fairly regular basis is going to be fine.