[Jeffy]

Rules of thumb will differ from team to team, but here is what I have used:
7 ft/s Slow
10 ft/s average
13 ft/s Fast
>16 ft/s Might be too fast to control

As for "precision" of a drive train.
Many things other than speed really effects this. If you build a stiff, tight and low friction drive train you will have very similar precision positioning ability at many speeds. Also programming an exponential throttle response can help the driver with low speed interactions. Trouble can be had when the skid friction needed to turn is high. This can cause the driver to have less control of the exact position during turning
For this year, I'd suggest you just build a robot with very little skid friction.

Just remember that top speed isn't really whats important. It's the point-to-point time for a robot that is important.