The nominal Battery Voltage differs between what type of batteries and servos you are using. Digital servos use up more battery capacity, and analog, use up less.. It also depends on how many servos you are using.
The servo operating voltage is 4-10 volts as you stated. It doe's not mean the servo will use all the voltage. It means you need a battery pack between 4-10 volts. Many people use 4.8 v battery packs. I perfer to use 6v packs around 2700 mah. If using battery packs over 6v, you will need a requlator. I don't think you are at that stage yet. Hope that makes sense.