Watching the bubble levels on the front of the rig, I set my Ground Control jacks down (older system, have to use the remote buttons) on my cribbing pads and level it fore and aft. Then I go inside, put my LVL-1 electronic level on the kitchen island and run the jacks from inside the rig until it reads perfectly level. The bubbles were very close, the electronic got it dead on.
Playing with boards or plastic pads under the tires got old the first summer we had the rig.