When I teach edge-jointing by hand, I am surprised by how many woodworkers (even experienced ones) evaluate their work incorrectly.
For starters, every board has a “true face,” sometimes called a “datum surface.” This is the one surface that you should press the stock of the square against. This applies even to machine work – electric planers are notorious for creating boards that taper across their width.
So you should mark one true face and always check your progress against that face.
Second: Tilt the square so one arris touches the edge (see photo above). Putting the square flat on the edge will put small errors in shardow. Ergo: You think you have a perfect edge, but when you get to glue-up you learn the gappy truth.
— Christopher Schwarz