Well, not that easy. While a 6 volt battery will work, I believe I recall reading that you may be reducing the life of your servo if you use 6 volts with it. Ask someone who knows for sure. I thought I saw this question posed today here at RCU, but can't find it now.

You don't say what kind of plane you will be using it in. If you are putting it into an electric plane, you don't need to worry about this. You plug your battery into your electronic speed control (ESC), and the speed control plugs into the receiver. The ESC will feed the RX it's advertised voltage. My GWS 300 ESC outputs around 5 Volts.