Karl:
Agreed with Lithium batteries, but I thought nimh batteries actually "liked" being deep discharged from time to time -- isn't that what "conditioning" a nimh battery does? (i.e. fully discharge and then recharge it?)
Actually, a bit of research showed that "deep discharge" is a matter of degree -- never discharge one below 1v per cell or risk permanent damage.
So, I'll rephrase -- never use rechargables with a device like a flashlight that can allow them to fully discharge, as discharging a 1.2v nimh to below 1v can cause severe damage to the cell. However, if a nimh will no longer hold a charge/run like it used to, a "conditioning" discharge in a charger that's designed to charge nimh batteries (maha c204f) could work wonders on the batteries.