I should mention that it's also possible to use laser level squares from the hardware store to square up a quilt. Here is a tutorial on using them to block a quilt, but they could also be used to mark a quilt for cutting:
http://www.dreamweavers-quilts.com/q...hort-tutorial/
It looks like a really neat method, and more accurate that just working on individual corners, but I don't have the space to try it out.