I've done the sew it on, then cut. Had wavy borders. It doesn't show that much (depending on how much it waves - if it waves like a flag, you've got problems) if you use a fluffy batting and tie the quilt, but boy, does it show if you are quilting on a frame. Now, I measure across, at least twice, use the average, add 1/4" to that measurement, pin down the quilt side (leaving 1/8" top and bottom overhang), sew, then trim the ends even with my 12 1/2" square up ruler. I use the extra 1/4" to make sure that the border end is perfectly square, not angling downward. Not the quilt police, but when I've put that much money and time into a quilt, I want it to be as perfect as I can make it. No CDO here (that's OCD in it's alphabetically correct sequence).