This thread is focused on NiMH batteries, but other battery chemistries aren't quite as forgiving. Checking the voltage before the flight works very well for batteries with a relatively linear sloped discharge curve. If you ever switch to A123 batteries, this paradigm goes out the window. These batteries have a very flat discharge curve at 3.3 volts/cell. From a fully charged condition at 3.6 volts'cell, they rapidly drop to 3.3 volts and stay there for 80-90% of the capacity. When they drop off, they drop off very quickly. Consequently, for the A123 batteries I've found that its more effective to keep track of the number of flights rather than rely on a battery voltage.

In reality, its probably worthwhile to do both. Know how many flights you should expect, keep track of the flights you've had, and check the voltage. If these two indicators diverge significantly, look for a problem.

Brad