I like using this voltmeter:
http://www2.towerhobbies.com/cgi-bin...&I=LXWW16&P=ML

I fly till its down 4.6volts on 4cell NICD pack. I guessing that if when i land if for some reason my voltmeter reads below 4.25 volts, then i was flying unsafe and should up my starting limit.

The downside is now i have to take of my wings to read the volts each time. I'm thinking of getting something like this:
http://www2.towerhobbies.com/cgi-bin...&I=LXPNU3&P=ML

Not sure if there are cheaper/better options out there.

Being an electric flyer, i know all too well how they packs can be unpreditable and go bad without warning. I would check every flight and don't assume you are fully charged at proper voltage coming off the charger.