I always try to use the length of fabric, because it does not stetch and distort an otherwise square quilt. But, if I am short on fabric, I will cut mine across the width. But, I am very careful not to let the binding stretch as I sew. Also, using a walking foot with WOF helps. I am also careful to measure the length of the quilt, and the width of the quilt, (through the centers of the quilt) and make sure the binding measures that same amount, keeping the quilt square.
Dawn