The problem is not open carry. The problem is the mental aspect... WHY are you "open carrying", at THAT moment?
The responses in this thread seem to indicate that they would specifically open carry, even in places where it wouldn't be legal off your own property, specifically as a "show of force". This is a dangerous attitude.
If you happen to open carry and you happen to be washing your car or you're open carrying and it's perfectly normal to go over and inquire with the occupants of the car, super. Have at it.
Doing it to INTENTIONALLY make them aware that you have a gun, to intentionally show them the gun, is a different mental state. The occupants of the car might never know the difference but it IS different.