Just sharing......I always square my quilt before I put on borders to avoid the wavy mess. I don't have a problem, typically, with making borders long and cutting excess but I make that determination with each quilt on whether I need to measure or if I can whack off excess. With wide (4"+) borders, as a rule, I measure and cut to exactness.
I staystictch only on the areas that have a seam so that my longarmer doesn't pull stitches when placing on frame. She tells me it's not necessary since she bastes it but I do it anyway because it still pulls on those seams and I've had to hand stitch a couple of seams after quilting. I'm all about prevention. Lol! Stay stitching does nothing to prevent wavy borders...