I try to make my quilt as square as possible, and then pick the same measurement for both sides. I cut the border fabric slightly larger (1/2"), but mark the true measurement on the border piece. Then, using Elmer's washable school glue, I put glue on the quilt edge, place the border on it to fit exactly to the mark, and iron the edge setting the glue. That way, no pins are shifting, the borders aren't pulling away, etc. I trim the 1/2" or so off after I sew and press. That little bit gives me an edge to pull on to make sure the border is fit to the right size. I always wash my quilts after sewing, and the glue washes out. This way, I do not get any waves on the edges, making it easy to quilt all the way to the edges. I quilt quite heavily on my vintage Singer 301 and I have smooth results this way. I don't know the science of this, but that it consistently works. Hope it helps. I also glue my bindings like in Sharon Schamber's youtube video Binding the Angel.
Linda