I rarely have wave in my borders. If I do, it's because of cutting the borders on width of fabric. If I have to do the WOF, then I will staystitch. That's the only way for me to keep it from waving.
Usually I square the quilt top up before adding the borders. I try to cut my borders with the selvage instead of width of fabric. Cutting lengthwise takes a lot of stretchiness out of the equation. I cut the border fabric to the measured length. Then I put markers at 1/2 way, then 1/4 from each direction on both the pieced top and the border fabric. Pin that accurately. Don't pull the pins out until you are ready to sew in that spot. Then I put a pin about two inches from either end. fyi...it amazes me how I can do all this pinning and get to the end and almost be short of fabric. Then, when I sew, I maintain a small amount of tension on the fabric to keep the top & bottom lined up together.