I like to square up at each stage. I think it makes each of the next steps easier. I square up my blocks and check for size so the sashing and/or joining goes smoothly. I square up my top so the borders go on easily, then square up after quilting. Often you really don't need to cut much off, if anything, just check for square. Sometimes the quilting will pull the quilt a bit so I like to cut my borders slightly (1/4" to 1/2") wider than the finished size so I have a bit to cut off.