I would avoid pins, actually any fastener might leave a mark that might be hard to iron out. Unless you are constantly taking fabric on and off the shelves, you shouldn't need one. After your shelves are organized, I would get 2 pieces of stiff plastic a bit taller & wider than your fabric. When you need to 'reshelve' a piece, put the 2 piece of plastic in the desired location, slide the fabric between, then remove the plastic pieces. Actually, putting a piece of plastic on either side of a bolt you want to remove will let you get it out without disturbing its neighbors & unraveling them.