I have been doing longarm quilting since 2002 and you get ruffles or waves in the border when it hasn't been put on correctly. Way too many people just sew a piece of fabric for the border. The correct way is to measure through the center and on each outside edge and then average the 3 measurements. Pin in the center, both ends and some more evenly spaced across the length or width before sewing the border on. This should be done for both ends and if there is more than one border on each side it should be done for each border. I can always tell when people have done it correctly, no fighting to try to quilt without waves or ripples.
This may sound like a lot of extra work when you can just sew a strip of fabric on but believe me it is worth it when you see the results.
Longarm