I'll agree that you don't want to make a habit of deep discharge of an NiMH which, by the way, is what daredevil is flirting with by not recharging very often after use. But that's not the whole story. From the page cited at the batteryuniversity (Buchmann is the same author I cited):


# High self-discharge - typically 50% higher than nickel-cadmium.
# High maintenance - nickel-metal hydride requires regular full discharge to prevent crystalline formation. nickel-cadmium should be exercised once a month, nickel-metal-hydride once in every 3 months.

The more important note above is the second one. Nickel chemistries tend to recrystallize in the cell by growing larger crystals. A larger crystal has less ability to make electricity because all to the interesting stuff happens - electrochemicalwise - at the edges of the crystal. Larger crystals have fewer edges and the battery eventually becomes a useless brick. NiCd does this is spades which is where the memory effect comes from but NiMH does it too just not to the same extent. I have a charger - a Maha C777 - that will discharge the cells to their lowest voltage and then pulse charge back to full charge. I do this once a month or so on my NiMH packs and nearly every time on my NiCd packs.

However, if you don't have this capability, it's best not to mess with it. It's very easy to go below the proper voltage and damage the cells. Figure out what the capacity of your battery is, daredevil, and then run the lights for long enough to reach within 10 or 15% of that capacity, then recharge. In your case, I'd suspect that four 30 minute commutes is going to be pretty close to your capacity. If the battery sits for several days (a week or longer), you should proably recharge it before use.