I usually fly my lipos untill the helicopter can't comfortably lift it's self into the air anymore without struggling. LIPO's do not suffer any negative effects from charging midway through a pack, so go for it. No sense in draining it right down to 6 volts. 7.4 volts is lower than what the micros FP brids are rated for (I think their nimh batts are 8.4 volts). So basically once you get down to 7.0 volts or so the helicopter probably won't be able to lift it's self under it's own power anymore.

Aslong as you don't go below 6 volts you're OK.