Do you big stitch with or without a hoop? If with, I would wait to bind it, you will need the excess backing to hoop the quilt to do the parts that are closest to the edge. If you quilt without a hoop you could bind it. I always wait until quilting is completely done before binding but your quilt is pretty well stablized that I don't think you will have any shifting. If you are concerned about the edges getting wear while quilting I always took my excess backing and brought it to the front and pinned in place to help protect the quilt edges when hand quilting in a hoop.