I make my backings about 2" larger all the way around - just in case. I use to leave more but, found I really didn't use that much more. When pinning my quilt sandwich together I pin at the very edge so there is no way it can stretch or move anywhere and it makes it easier and more accurate when I cut it to size for the binding to be attached. I quilt within about 1/4 - 1/2" from the pins and that makes it easier for me to hand stitch the binding to the backing. I use to flip the back over the front and sew that down for the binding, I stopped that.