I ususally prewash my fabric as soon as I get it home. I recently was making a small quilt using charm squares - figured they weren't prewashed, so I didn't wash the co-ordinating fabric. Cut that fabric, pieced my squares, then pressed. The not prewashed yardage shrunk just with pressing. Yikes!! I'm sticking with prewashing everything - even any more charm squares I may buy in the future.