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
Also, the tilted surface of the square can, somehow, catch the light and channel it into the edge. This emphasises the errors much more. Held vertical, the blade could be casting a shadow onto where you are trying to see the light.
I always thought when my square squealed when drug along the surface, it was in… 😉
(eh, not completely kidding. I have a square that actually does this. The tool itself is a cheap machinists-type and not actually square, so I no longer use it. It goes “Wwhhoooaaat!”)
I’m good with that. I’m right at home with a little tilt here and a bit of lean there, besides even my ex told me I’m a little off kilter!