If you’re a new or home machinist, learning how to “square your parts” is one of the first things you should learn. There’s quite a few common tools that a machinist should have in their toolbox, and a Machinist Square is one of them.
Learning how to machine a part on a manual milling machine should be one of the first things in school. Eventually you will need to make parts with tighter tolerances, and flatness/perpendicularity are a big part of it. If your parts are trapezoidal because of tool deflection, I can guarantee that your parts will be rejected.
A good machinist square should be perpendicular/flat within .0002″ tolerance, and should be periodically calibrated so that it stays within spec. However, some squares have a different rating that depends on how accurate they are. A & B are the most common, with B being for the average consumer that doesn’t need extremely close tolerance parts.
A is the higher grade, and while it will cost more, it is what you will want for machining. Don’t skimp out and get the cheapest one you can find. You get what you pay for, and in CNC manufacturing these days, you want every advantage possible.
If you drop it and can see a dent or bend in it, it’s worthless. That’s why it’s critical that you take care of not only a machinist square, but all of your machining tools.