True goldslinger, very true!
I wouldnt let them get below 5V either, although i've never reached that as i got my DX7 before that .

Although the CX2 TX's (LP5DSM) are really power-efficient, once there's too little voltage to run the voltage-regulator and fill the capacitors of the 2.4Ghz unit, it'll show some strange and dangerous (for your CX2) behaviour!