It’s -supposed- to be linear over the belt, but I doubt it is, and @RichCournoyer’s data shows that. It’s a matter of degree. If your belts are REALLY tight, you can adjust for that. If they’re REALLY loose, you can adjust for that. There are some nuances though. After three adjustments (post stretchy) I’m at ~38 in X and ~37 in Y, but I started at ~42 for both. If I hadn’t adjusted, that would be a fairly big error, and on my machine, that’s fairly even (checked across 12" in Y, and 6" and 12" in X) - but I don’t worry too much about short distances (an inch). If you are cutting pieces that need to fit together, and do them at the same time, the errors are less important because the errors should be the same magnitude. If you’re cutting to fit a part you cut six months ago…more so…depends on how big the error was then versus now.
If you’re getting accuracy you need for the parts you’re doing, don’t worry about it. For example a sign, obviously, doesn’t matter so much in absolute dimensions, you just want round circles.