This is great advice and makes all the difference. The ONLY time I will cut across the width (selvedge to selvedge) is with a directional print.
Also, always measure across the middle and sides to get an average, then cut the borders exactly to that size. I allows you to ease in any extra or stretch a bit if needed to make the quilt square. Once your border piece is cut to the exact measurement, find the center of your quilt side and match it to the center of the border strip. Start pinning from the center and work out to both corners.
Also, make sure you are not stretching the border as you sew,