[Mr V]

I'd say there is no universal tire pressure that will work well on every robot.

The contact patch of the tire is dependent on the ratio of the load on the wheel vs its pressure. A light robot with lower pressure will have the same contact patch as a heavy robot with higher pressure.

I'd also suggest that it may be a good idea to run different pressures for the wheels that are dropped vs the other wheels assuming you have a 6 or 8wd with a dropped center(s).