When I make a quilt I make sure the outer borders are sewn on squarely whether they are mitered or not. After I layer my quilt and baste it (I like the Elmer's school glue method) I stitch around the outer edge with a long stitch. This keeps the layers from separating while I quilt it. Then, it holds the layers together when I trim it. I am careful to use a large square ruler when doing the corners to be sure they stay square. This works well for me. I generally sew the binding on the back by hand.