Tartan, thank you so much for addressing this topic! I've been quilting for many years, making mostly art/wall quilts and I think I'm finally able to square them up, but they always need adjustment.
I use a gridded interfacing on my design wall, on top of the batting that covers the design wall. Many people use a table cloth that has squares to use as a grid. I pin my quilt on it, spray it with water or Magic Sizing and steam press. I also make the border larger than I want so I can trim it if necessary. All of that is done once the top, batting and backing are assembled. Because I make art quilts, I add an extra layer of felt to make the quilt lay straight.