Generally the best way to square up a block involves using the interior seam lines of the block and a ruler.
http://www.quiltuniversity.com/squaring_up.htm It's not as simple as plopping a square down on top of it and slicing the edges off. I don't see how pressing a piece of freezer paper to a finished block would result in accurately squaring it up at all and would just add extra steps and greater chance for error imho.