I squares up throughout my quilt top. No matter how square it is before quilting, I still get a little movement and I square up the sandwich before putting on my binding. I usually put a plain border on my quilt top so that squaring up doesn't cut any points off my blocks.
I lay my quilted sandwich flat on the floor. I put my large 15 inch square in the corner and line up all my long rotary rulers down one side of the quilt. At this point I can mark that edge or if I am feeling confident, I can slip my cutting mat under the sandwich and trim. I do the other 3 sides the same way. I am only trimming a smidge away to get a nice straight edge to attach my binding to. Before adding the binding, I fold my prepared sandwich end to end and side to side, matching the corners to make sure everything measures the same.