When I am squaring up a block comprised of different elements, I place my ruler on the center seam and measure out from there. Do this on all 4 sides. When making HST's, I center the ruler on the seam line again and measure to the outside edge on all 4 sides. In both cases, this helps with making sure your block is centered as well as not clipping off important points. I believe in squaring up each element or block as you go along. Generally speaking, you then don't have to square up your entire quilt after it's completed because everything you have put together is already square.