What I've learned so far about making square boxes (like for the built in bookshelf cabinet doors) is that all of the sides need to be cut perfect. 1/8" off is horrendous. For the cabinet doors, the sides are longer than the tops. Which means the sides need to be identical. The electric miter saw helps greatly with this: a cut can be made at 45 degrees in a few seconds. However, ensuring that each board is identical length takes a lot of work. Cutting too long and then trimming down slightly works just okay.
Miter table benches/fences can be bought, but I figured I could get a lot of the functionality of a miter fence by making one and save some money. So I did out of some scrap MDF I had from a previous project..
Now, all I need to do is get one board cut to the exact size I want it. Then, I adjust the little block on the left side and can then cut identical length pieces over and over again without ever measuring or remeasuring.
So now, what I'm going to do is cut all of the vertical pieces of wood for the cabinet doors with the block set at 30". Then, I'll move the block to cut at 16.75" and cut all of the tops and bottoms. Then assemble.