If you jack it up and try to turn the wheels on an open diff. you end up spinning the spider gears in the differental and it does not give you usable info. Whichever wheel you spin the other turns the other direction and the driveshaft does not turn. It would work on a posi or locked locker.
As far as which is easiest I guess it's just a matter of preference. Pulling the cover would be 100% accurate. Doing it this way just saves jacking it up, the mess, the time to clean gasket surfaces and resealing and refilling.