I wouldn't worry about it unless your using a high-speed buffer to compound your whole vehicle. Under those circumstances, you can identify areas of your vehicle's exterior that might need extra work.

If you're just waxing your car, no, I wouldn't worry about it. If you're hand buffing a scratch off your car with rubbing compound, no, I wouldn't worry about that either.

If you're repainting your car yourself and wetsanding it, then buffing over it for that sparkling finish, then you might want to take it under consideration. ;-)

Truth be told, there are few reasons that people other than professionals would need to guage the thickness of their paint.