I square blocks as I go. Once everything is sewed together, I check all the corners of the quilt that they are square with my 22.5" square ruler. It helps to have a long run on either side of the square but I can't say for certain that all my quilts are exactly square. Only by folding them in careful halves does that show. I do staystitch the edge before quilting and tend to use bias cut binding which is very forgiving. Nobody has ever complained to tell me that a quilt is not square.