The guiding principal is that your MV shouldn't look like the original base vehicle. If covering the wheels helps it look more like whatever it's supposed to be (cow, head, couch, etc), then that's preferable.
I'm giving this a nudge over to the Transportation board, since that's a better fit for general questions and problem-solving. The year-specific MV board is more for announcing and discussing fully formed projects that are coming to the desert that year.
It's not a specific requirement but it's one more thing that helps. Look at some of the all-time great M/Vs like Flash & Tom Kennedy's Whale, the La Contessa, or the Catbus. If it's something that isn't supposed to have wheels, it's so much more magic if it doesn't appear to have them.
Too much is not enough, and when in doubt double it.