When I sew the borders on I go to a slightly larger stitch than for piecing. Because you are sewing long seams sometimes the machine stitching tightens in places because of the way you guide the fabric thru or start & stopping, so when I loosen it by 3 stitches it takes care of that happening. I try to sew without stopping and simply let the machine pull the fabric thru. I have never had a wavy border