I use my omnigrid rulers -- I use the largest ruler that works for what I am squaring up -- I have a 20" (or is it 20.5") for my quilts and bigger projects. There are some neat little corner things on the ruler that I match up with corners of the blocks if I don't have HST's to help square things up. I like the bigger ruler for squaring the quilt because it gets from the edge to the blocks.