well it usually isn't necessary but I have taken my largest ruler (20 1/2") and placed it in the corner. matching lines of ruler on some seams and seeing if anything needs to be trimmed. I'm talking maybe 1/8 inch to be cut off. I do that for all corners.
now if you mean wetting the whole quilt and "squaring" it up that way, I've only done that once. out in the backyard with a yard stick and long tape. too much trouble for me.