I previously used Li Ion's but now only use LiFe's for Rx batteries. If the servos are rated for 6 volts, then there is no need to run a reg. which is one less failure point.
As for checking what's left in the pack, contrary to what I have read here and on other threads, I check mine by voltage. I ALWAYS check the voltage IMMEDIATELY after landing. My stop fly voltage is 6.3 volts under a 1 amp load. I have been using this system for 4 years and it has been 100% successful. I monitor how many mah's are put back in when charging and there is always more than 25% left.
After reading another thread recently, I checked the batteries using a battery checker as was suggested as the only way to monitor LiFe capacity. Using my method, I get 4 flights before my minimum voltage is reached. Using the claimed, ONLY WAY to check capacity, after 2 flights my checker showed 25% capacity remaining. When I charged them, it only put 220 mah's in. They are 800 mah packs.
So I will continue to check by voltage.