I have to agree with minic@HYPER69, castors are not a good idea. Imagine having a 130 lb robot hit you when only 2 wheels will resist turning. Last year there were a few robots that had castors that tried to make it up the ramp, and the only ones there were able to do it were the ones that were smart enough to drive so their powered wheels went first up the ramp. A 4 wheel design works just fine, and a 6 wheel can get a bit tricky if you don't know what you're doing. As it's been said before, having castors as part of your drive train is the easiest way to identify a rookie team.
This team tried to make it up the steps using a 2 drive/2 castor robot, it didn't work
Here