I put a Voltwatch batt monitor on all of my planes... just bang the servos around before and after each flight and see how low the meter drops... when it starts getting down in the low region, stop and recharge them. I also put an AirAlert on all of my planes too... makes recovery in the corn or brush much easier (we have both around our field). The AirAlert also has an audible low batt alert, but I use it mainly for recovery insurance... Voltwatch is a better monitor for your batt pack.

http://www2.towerhobbies.com/cgi-bin...&I=LXSZ87&P=ML

I've heard from several sources that NiMH batts should be stored with some charge, but I don't believe NiCds care either way. I always try to charge my batts up when I get back from the field, and then usually peak them (about a week later) just before I fly again.