RE: New Hitec Aurora 9-Channel 2.4Ghz Radio System
While it is preferred to use a 6v pack with Spektrum Rx's, I don't believe it is Required as a minimum. There are some servos that can't handle 6v. It is more important to be sure the battery you use has more than enough Capacity to handle the Current Draw. There are tools, like a Wattmeter, or the Hangar 9 Current Meter, that can tell you how much Current you entire system draws (or just one servo), with all the servos under Maximum Load. Even with 4.8v, you are less likely to have a Brownout if the Capacity of your battery is more than enough to handle all your servos Stalling at once. You still could get a Brownout with a 6v battery is it's Capacity was Marginal for your system.
I am speaking in reference to the comments about Spektrum Rx's as I do not know about the Hitec Rx's.