Good advice already here, I am just an echo.
1. How square can you really cut or shape once you have a perfect mark?
2. There are plenty of cheap tools out there more square than I can cut.
Derek Cohen I am not. He functions at a level I am not likely to achieve in this lifetime. He has talent, I am trying to build what skill I do have. My best square, long term, real life, has been an aluminum casting roofing square, a speed square. I bought it in the 1990s for 10 or 12 dollars. I have dropped it off several roofs, many of them second floor and one of them a third floor roof. I can still lay it on a factory plywood edge, make a knife mark, flip it, and make another knife mark more parallel to the first straighter than I can cut. My idea of caring for it is to set it gently on a concrete floor rather than dropping it from waist height. Over the course of decades in real life I am not happy with any of the wood insert squares I have purchased. I have three combination squares that have become junk combination parts with still accurate steel rules. Combination squares wear out. There should still be a good steel ruler in there, but they (the part that makes a square) wear out.
When I need a new square I go to Lowes-Depot, grab one of those 2x2 foot pieces with a factory edge showing and then head to the tool department with my marking knife to pick a new square. Often cheaper to buy a 4x8 sheet rather than a 2x2 offcut if you can tote it home. Someday if I become good enough I might invest in a square not from the home store, but it will make me nervous to not have a factory edge and a marking knife when picking the one I want.