I just want to make it clear that there is a difference between (1) using the serpentine stitch on your machine and (2) quilting wavy lines with a walking foot. The wavy lines are made with a straight stitch and a walking foot, but you move the fabric to the right and left as you sew. With serpentine stitch, you are basically sewing a straight line but using a stitch that creates waves to the right and left of your stitching line. Both are useful, but the look they create is quite different.
Hope this is clearer than mud......