I don't think it's about being level so much as not being "banana-ed." Back when I was sailing a lot, we'd call the situation where the fore and aft stays were too tight "banana-ing the boat" because of the very slight pushing down in the middle and raising up of the ends that resulted from overtightening the stays, resulting in a slight banana shape to the hull. The same thing can happen by overdoing the muscle power when screwing down the jacks. The effect is too small to see but enough to affect the door. I don't think any damage occurs, though, just inconvenience.
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator