Originally Posted by shannow

I'd say it's negligible.
Sorry I just did this video for you question the quality is crap and it's loud (turn the volume down a bit for the squeal of the motor):
https://www.youtube.com/watch?v=DATXwtVJoSY
The car is on water not touching anything else. The test speaks for itself.
I tried several axis. I can feel resistance (very light) on the vertical axis with no rotation direction preference but pretty much none one the other two.
Certainly no movements on it's own. So changing direction shouldn't make any odds or to me at least it's negligible.

I'm open to any theory/counter theory as I once contemplated the idea too.