even when the batt. is fully charged the light will "dance" a bit when applying a load,using the sticks on the radio.
I am pretty conservative with battery power levels, when the voltwatch is on the last green light its time to put that plane in the truck, and pull another out there just too valuble to me to risk it.
of course not everyone has more than one plane to fly on a particular day(I usually bring 2 to the field,sometimes 3 if its going to be a all day thing)but having a extra reciever batt isn't a bad idea if you are planning on flying more than 3-4 flights in a session,I have found that a typical Futaba 600mah batt. is good for about 3-4, 12-15min.flights before its drained down to the point of a safe level to fly,on most of my planes,not all but most, I have gone to a 1,000-1,500mah batt. because I have more servos to drive and they drain the smaller batt. down too fast for more than 2 flights.
with alot of transmitters there is a low voltage alarm or at least a safety meter to let you know when there out of juice,I use the voltwatch to check the power before the flying day.

I am going to permanently install them in stelth locations on my planes I ordered a couple more just for that purpose ,ie. inside the cockpit area so I can check the voltage when the planes are assembled through the canopy with WWII planes a voltwatch doesn't look all that great stuck to the side of a fuse but I am sold on the value of a on board meter.