I always wash all my fabrics--including the poly cotton backings that I use. Two reasons--shrinkage and bleeding. Seveal quilts ago, I needed a piece of blue for a backing and could only match it in a poly broadcloth. Thank goodness I washed it first because that sucker bled like a stuck pig and would have ruined my beautiful quilt on the first washing. Yes, I mix polycotton and 100% cotton but EVERYTHING gets washed first!!