I measure quilts down the center with border fabric, lightly smoothing it down. Then I do the same across the quilt, divide the quilt edge in thirds and the border pieces in thirds, match pins then sew it on. Been doing it this way for years, works for me. I square everything up as I go.

I also agree with quiltingshorttimer That's the way I was taught. I've never had a problem with my borders being wavy.