[SpeedFreed]

The steps I always take to make sure all wheels touch the ground are:
1) I don't make perfectly rigid drivetrains (on purpose!). You shouldn't be able to notice any flex, but it should be there in ever so slight amounts.
2) Mecanum wheels that flex, such as Vex Pro Mecanum wheels, actually have a small amount of give, so they could be ever so slightly off.
3) Diligence. I always use a CNC machine or mill with DRO to drill holes in my mecanum drives to make sure they are exactly in the right spot I do this for many other parts of the robot too).

But I can't imagine it would be all that different from a typical 4 wheel mecanum, just doubled.