Dual rate adjusts the travel that both the left and right directions can steer. For racing it's very useful. When I used to race onroad nitro, sometimes the track would have so much traction I had to turn down the dual rate, limiting my steering throws, but getting me around the track quicker. If you're bashing with your truck, I'd start with the maximum steering throw, run it, and adjust from there if necessary.
Regarding your wheel pointing direction, grab a set of calipers and check to ensure the turnbuckle lengths are symmetric between the left and right side. This goes for steering links, lever pushrods, and even shock rod lengths depending on how OCD you are (I am). Once you've done that, very likely you only need to trim the steering so that it is centered. It is a dial on your radio. I recommend starting with 0deg toe for the steering wheels (both wheels pointing straight ahead) and adjusting from there. Most of the guys on this board are probably running slight toe out in the front wheels (both wheels pointing slightly outward).