I generally square up my quilts after I've quilted them, and of course, before I bind them. I find that the quilting process alone can get my quilts out of square (especially if I'm not quilting the border first). I only try to square them up if I'm not using a pieced border though. If my border is pieced and I can't lop off any of the outside because of points, etc, I just try to ease it square the best I can while I'm basting it, and hope the quilting doesn't "skew" it too badly.
Maybe someone here has a great idea how to square a quilt with a pieced border. I'll be checking back in the hopes they do. :D