I had this same question about our 6wd setup last year. I found that the offset distance varied, based on a couple factors. What tread you were using, how much "scrub" you wanted, how much "teeter" you could handle. In the end, it came down to the drivers feel for the robot. We used a series of 1/16" shims to fine tune the handling. We would go drive around until a battery died, then go back and add or remove a shim. When we found what worked best, we made a permanent shim and never touched it again.
Once it's built, play around with a couple different shim heights. You'll be amazed at the significant drivability changes, with such a small height different. Oh, and make sure this is done on a similar carpet to the playing field. Otherwise, all bets are off.
BEN