When we have to join pieces to make the binding, regardless of the method, there will be 4 layers of fabric at the joint. When we choose to sew them straight across, all of these layers 4 on the front side of the quilt AND 4 on the back side of the quilt will fall on the same spot and will be bulkier than the diagonal joint. When we choose to sew them on the bias, there will still be the same number of fabric layers, but the joint on the front of the quilt will be offset from the joint on the back. It is now my preferred method.