Hi GPS1,
The official recommendation is to charge at 1C. With 4200mah IB cells due to the risk of explosion, this became a must due to safety.
With 4600mah, the official recommendation is still 1C, but there has been no reports of them going bang. So what you ought to do, is check with the club you are racing at, as they may still require you to charge at 1C, so do so. If not, then it is your own decision to charge at whatever rate.
There is really no need to charge at 6amps tho, you will just over heat the cell and cause it them to age very quickly. If you are worried about speed of charging, the new "breed" of cells seem to hold charge well (I am using EnerG), so I recommend partial charging at home (2000-3000mah) and then peak them off at the track. You don't really need to fully charge them, the car will only use 2000mah per run - so knock them off at 3000mah and they'll be great and the cell life will be hugely extended.
Personally, I am charging at 5amps now, on a Pulsar 3 on linear with delta peak at minimum, 5amps on 4600mah is only 1.09C, and I am feeling that my EnerG cells are awesome for punch and duration & take a little less than an hour to charge from equalised.
Chris