Nice method for box making. I make picture frames so the router won't work, so I use a Grizzly miter trimmer to get exact 45s. Cut near to length, then trim one end (about 1/32 of an inch), then but the other end against a stop block to match length and trim the other end. Important not to make the length more than about 1/16th inch long because the trim should be very thin to produce the best result. But for Boxes, pre cutting length sound like a good way to go.
The only thing is that mitered corners are quite weak, so you'd need splines, biscuits or some other method of reinforcing the corners, not just glue.
The more I do, the less I accomplish.