the servos run on 4.8v to 6v, just depends, a lot of the servos ive seen will run on anything in between and will work faster the more volts you give it up to 6v.....

Operating Voltage: 4.8-6.0 Volts
Operating Temperature Range: -20 to +60 Degree C
Operating Speed (4.8V): 0.17 sec/60° at no load
Operating Speed (6.0V): 0.14 sec/60° at no load

see it operates anywhere between 4.8 and 6 volts but look at the speed, 4.8 its .17secs and 6volts its .14, .03 seconds faster....not sure if you could tell the difference with that small amount but i guess every little bit helps.