If you'd like your project to turn out as strong, solid, and lasting as if a professional woodworker built it, do what they do...use your tri-square to check for squareness after each cross cut. Edges on boards that will be edge-joined must be absolutely square, so carefully check all sides. If a cut is not exactly square, use a block plane to trim. It takes a little extra time, but the improved results will be well worth it.
Say you’re building a deck in your yard and have to cut a 10-in. thick post in half. How do you do this with a circular saw with just an 8-in. blade? Make matching straight cuts on opposite sides of the timber. Set the thickness guide to just over half the woods width.