I'm careful to keep things square as I go along so I've never had to square up a whole quilt. I think that and using a walking foot and not pulling the quilt while quilting is key. When I lay my ruler along the edge it is square. After I quilt I sew a narrow seam all along the edge of the border to hold the sandwich together at that point. I then trim away the excess backing and batting. I always have some sort of a border on my quilts so I use a 3/8" seam to sew my binding on with with 2.5" binding folded in half. Because I have a border I don't have to worry about cutting off points or elements of the piecing by using a 3/8" seam which happens to be the width of my walking foot. It also fills the binding up nicely with batting. I know that it is unconventional but it works for me and I like the look. To get nice mitered corners I then stop 3/8" from the end before turning.