The only way to truly determine you're personal cutoff voltage for flying, especially given you're apparent concern over the condition of the batteries, is to develope a discharge curve for your battery. Its easy, but time consuming. Here's how.
If you have one of the DVMs made for hobby use, just use that meter.

If you don't have a loaded DVM, you can go to Radio Shack and buy two 1W 10 Ohm power resisters. Hook one end of the resistors together by twisting them together. This will put an approximate 240 mAh load on your battery. Hook it up across the battery terminals so you can check the voltage with a volt meter. Preferrably a digital volt meter. Set it up where the resistors get plenty of air, and don't have it sitting on anything flammable. The resistors will get hot.

Now hook up your rig (DVM or resistors) and start plotting the battery voltage against time. Check the voltage every two minutes. Be precise.

Your voltage curve should look something like the graph below.
Brad