Dave,
With the 9202 servo, which is a good coreless servo, the 6.0 battery is very much recommended. The 9202 is a little older, less expensive, and therefore a little weaker and slower than others and the extra voltage will enhance this servo's performance noticeably. Be sure to check you rudder servo occasionally as it receives quite a bit of punishment. A new gear set every 50 flights or so on the rudder is a good way to spend $6.00.

Mark