I never have wavy borders. I'm not the best quilter by far, but for some reason (knock on wood) my borders always lay flat. I don't know what I do 'right' to make them lay flat. I know I don't measure my quilt in the middle. Never have. I don't measure it anywhere. I just make sure I have enough border (always a few inches extra) to go from one end to the other and sew it. And I never have wavy borders. My LAer told my LQS owner that she is very impressed with my sewing. I was shocked because I don't do the things that everyone else does - like measure the middle of the quilt and make your borders that size. But it works for me and I'm not changing.