There's really only two reasons to do a Test Spot to each body panel on a car.
1: Some panels may have deeper defects.
2: Some body panels may have been repainted and thus the paint could be harder or softer than the panel you did the initial Test Spot to.
I don't do either. I practice and preach to do the Test Spot to a horizontal panel that you can look down on as this makes it easier and more conclusive to judge the results. Also, the horizontal panels tend to be the worst condition as compared to vertical panels. So what works on a horizontal panel should work on a vertical panel.
And of course, as you work around a car, IF you see something changing, then of course, stop with the current process and test a different process for that body panel.
To simply do a Test Spot on every body panel would require some time, maybe an hour or so and I would rather be buffing out the car and getting to the finish line than messing around doing Test Spots to each body panel when chances are VERY GOOD - the car has the same paint on each body panel so the results from testing on the hood, roof or trunk lid means you'll get the same results.
Even if a body panel has been repainted, as long as you're using quality products, matching the right pad and tool for the job, then it's quite possible the process you dialed-in on the original paint will still work on the repaint. But this is why every detailer should have good lighting so they can see if something is changing as they buff.
And of course, each person can figure out an approach that works best for them.
-Mike