I square up as I go and take a couple of measurements on my borders. I pin, pin, pin my borders on, being careful not to stretch the tops edges and distribute any discrepancies between the border and the top between the pins. Ease in any fullness as I stitch the borders on. Also if you do a lot of machine quilting on the bias of the top, that can stretch it as well. If you are adding more then one border, sew the borders together before adding them to the top.,