The modern Futaba TX s are designed to work (meaning there would be no change in signal quality and power) down to 8.4Volts.
However, any voltage reading below 9.6V is dangerous because it will fall below critical levels much faster than it took it to get down from 10.9V to 9.6Volts. (NICD and NIMH Discharge Characteristics)

It is a wise thing to keep the voltage reading above 9.6V on the TX screen.