I haven't done it this way myself (yet), but using a laser square from the hardware store looks like an accurate way to do it. Here is a link:
http://www.dreamweavers-quilts.com/q...hort-tutorial/
I know it's for blocking a quilt, but it would work for squaring a quilt also.
For me, squaring a quilt sandwich by cutting has always been a bust. What I do instead is use a Sharpie black marker to mark the corners. That way, if I need to make adjustments, there is less squirming around of the quilt sandwich while I am working.
Unlike most people, I do not cut the edges before sewing on the binding. Instead, I use the Sharpie line as as virtual edge, lining up the cut edges of my binding strip with the Sharpie line. Only after the binding is sewn on do I finally cut the quilt edges to size. (My only caution about this is to be *very* careful at the corners; you do not want to nick the binding.) Doing it this way seems to keep my edges from stretching, and I also don't have to worry about a fabric edge folding under.