I have never found it necessary to square up my entire quilt. I measure the quilt top in the center and 1/4 in from each side. I take the average of those measurements and that is what I use for my border. I sew the side borders on first and then the top and bottom borders on next. This also works if you want to miter your corners of your border. Just take the measurement and make that what you sew to the quilt leaving extra on the ends for the miter. Lay the quilt on a flat surface to work out your miter. When I'm pinning the border on I divide the quilt into fourths and the border into fourths and match it up. Sometimes I have to do a bit of easing in with pins. I've never had a quilt that didn't lay straight and flat when I was done quilting and binding it using this method.