[EricH]

DeepWater hit on exactly why you'd want the CG in the back (or front, depending on your objectives). Now, here is why you probably don't want it in the middle:

The rock is a lot harder to control. You have, in effect, an inverted pendulum with a barrier on each side. Balance it right, you're fine. Tap it lightly, or blow air on it, and it goes crazy. FRC robots get that kind of tapping from normal handling. Now, let's say that you want to pick up an object and put it somewhere with a reasonable degree of precision. You're rocking probably about 1/8" in either direction from dead horizontal, due to the "height" of the drop with relation to the end wheels. So you have to allow for a 1/4" variation in location, just in normal rocking--and that's not including the multiple other places that you could get an error.

So you really don't have a huge amount of precision, while by moving the CG back about a foot, you still have that amount of variation, but you know that it will be momentary at worst, and you'll end up back in your "normal" position.