everyone uses 6 cells because that is the maximum legal limit for racing. Sometimes I use 7 cells for fun, it really makes the car come alive and shows how good of a driver you really are.
The only thing you need to pay attention to is your motor. It will run a little hotter because of the higher voltage and it will wear quicker than average. ESC temp can be an issue if you don't have a good esc (gtx, g11, qc2, etc).
I would reccomend that you not run 7 cells if your motor is less than 12 turns - too much heat, just buy an 8 turn instead.