Have had to take a seam in a couple of quilts I've done with wavey borders.
On a king size bargello I made for son's wedding, the borders where 10" wide and mitered (a free Jenny Beyer pattern)--she had you put all borders together first and then miter. worked fine that way. Also, borders cut from length of grain tend to stretch less---don't do many mitered borders, but any time I do borders that are 3.5" or more I always cut them length of grain--seems to help prevent waves.