I always did it that way-sew on a longer piece and trim. Never had a wavy border. Then I read that it's not the right way so I switched. I figure with my luck, the next time I did it the old way I'd have a wavy border.
Do you sew them on with the border fabric on top? The feed dogs will feed the bottom fabric every so slightly faster. When that happens you'll get waves.
Are you supposed to keep the border fabric on the bottom?