Gary,
I have a BigFoot system (only four jacks), but works the same. Like others said, sometimes you need some blocking. Blocks can be heavy, and the jacks can slip a little sometimes. I made four of these stands, with a lip around the top to keep the feet from slipping off. Most of the time I don't need them, but some times I camp in sites that are way out of level. Example is the picture in my tag line, when the Monty was parked at the house getting ready to go out.
This is version 2 of my stands, made out of steel, much lighter than version 1 which was made out of wood.