ORIGINAL: guver
The low voltage ''resting point'' for lipos is 3.5-3.7 volts per cell. For power systems there's no useable capacity below 3.5 volts , but since you are using in radio app with low current then there will be some useable capacity below 3.5 volts/cell. I'm not saying that you should use this extra capacity.
Actually once the voltage drops below 3.4v you only have a few minutes left with the TX.
Gorish: JR and Spektrum TX's seem to be tolerant of higher voltages.
I've been using LiPo packs with them w/o problems as have many other members at our field.
However I gave a friend a brand new LiPo pack to use with his Futaba, and it blew the regulation circuitry on the third day of use.