The answer to this question is a bit fuzzy...[8D]

The only real issue with servos and voltage seems to be related to some Futaba stuff that IIRC, is mostly used for heli's in conjunction with their gyros. Those say 4.8 volts only and they mean it.

Others will say "officially" that their servos are limited to 6 volts, which is a bit confusing since a 5 cell NiCD/NiMH battery fresh off the charger will usually hit 7 volts and by the time they get down to 6 volts, they need to be recharged. 4.8 volt batteries are pretty much the same in that the usable voltage range is such that when they get down to 4.8 volts you better charge 'em.

Now for the reality; most servos with the exception of those few which are restricted to 4.8 volt systems will live quite happily on 6 volt batteries which actually run over 6 volts in practice. Some of the Li-Po/Li-Ion setups which crank out 8.4 volts fully charged and usually play around the 7.4 volt range are run with voltage regulators to cut the voltage to 6 volts. Some gasser ignition systems will not take much over 6 volts and have to be regulated even with a NiCD/NiMH 6 volt battery. Some servos, particularly as they get a bit older and the pots or gears start to wear, will exhibit a bit of "nervous" activity on freshly charged 6 volt batteries but it's not usually much of an issue. Just blip the servo once or twice and they'll stop jittering, usually that's a sign that the servo could stand a trip to the service center.

Now the hot new setup for out purposes is the A123 battery which is a Lithium/Iron nano-phosphate technology and puts out a pretty constant voltage through out it's discharge curve at about 6.6 volts. Also a cheaper alternative is the LiFEPO4 battery which is the same chemistry of Lithium/Iron phosphate but is not the nano-phosphate technology which is patented by A123. They work in the same voltage ranges, but do not have the output capability of the A123. These batteries charge in 20 minutes, no fire hazards like those associate with Li-Po and to a lesser extent, Li-Ion batteries and just really work well. Out of all my models I have one analog servo on a glow model that gets the nervous "twitch" when first turned on with those batteries. Everything else is perfectly happy with the A123 batteries.

Okay now that I've probably put you to sleep, the short answer;

Run good quality 6 volt packs unless you have either one of those 4.8 volt restricted Futaba servos or an ignition system which won't take anything over 6 volts. Don't fret about servo life, it may be a bit shorter but I doubt you'll actually notice it and the manufacturers are real good about fixing them if you have a problem. It's more important to keep your computerized radio system happy than worry about a bit shorter servo life and as a bonus, you'll get faster servos with more torque at 6 volts vs. 4.8 volts.

Clear as mud????