Originally Posted by cyccommute
The more important note above is the second one. Nickel chemistries tend to recrystallize in the cell by growing larger crystals.
Keep in mind that the quickest way to grow crystals around the electrodes is to discharge a cell below 0.95 volts per cell. If you run a NiMH battery down below there, you will incur irreversible crystallization around the electrodes and the cell will be permanently damaged. The more you do it the worse it will be.
So while running them down is probably OK, running them dead is worse than not doing anything.