I would wash everything before you sew so if anything is going to shrink it will do it before you sew. Cotton shrinks more than fleece so the only way to prevent shrinkage is to wash in cold and lay flat to dry. If you wash first you don't have to worry about shrinking nearly as much, if at all, because it already has. Washing before hand also gets rid of any chemicals or extra dye that may be on the fabric. I always wash my fabrics before I sew anything and get extra yardage to account for shrinkage. Also I'd worry more about the towel shrinking than fleece because fleece is a synthetic fiber.