Current is what destroys a servo,not voltage. A servo rated for 4.8 volts can be used on 8.4v if it is not in a heavy use situation. The higher voltage will draw higher current if it is worked hard and that is what burns it out. The tail rotor servo in a helicopter is working hard all the time and is subject to hard use. The same servo can be used on say the ailerons of a foamy and not be bothered.
In any case I would never exceed its specified rating.
.If you use a stabilizer in your aircraft,keep in mind the control servos will likely be working all of the time like a hellie tail servo does