I never experienced any bump steer on my arena, so I haven't shimmed anything. I think bump steer affects pillow ball suspension the most. C hubs are a bit different, and do succumb to bump steer as easily.
I wouldn't worry about the bump steer until after you get it on a track and test to see if there really is any bump steer.
The baseline is thicker if front, and thinner in rear. I'm running a softer set up (40wt front, 30wt rear) but it works well for me. Most run 60 wt in front and 40/30wt in rear. It's all setup and driving style, but I would go with any one of those to start with. Also, check out the CEN team driver's setups.
EDIT>>>woops, I thought this was the truggy forum for a sec hehe!
Sorry outtafocus, I haven't run my buggy yet so I can't help you out...I tried!