After I finish piecing a quilt top, I sew a basting stitch all around the top before sandwiching it to do quilting. The reason I do this is because I don't always do my quilting right away. Sometimes I stack the pieced tops and wait until I have several to quilt. I have found that sometimes piecing around the edges will come apart because I don't lock the stitches. The basting stitch prevents this from happening. I don't baste around the backing unless it is pieced. If I only have one seam in the backing, I press it open and baste across the open seam line at the edges. I hope this makes sense.