Supposedly, cycling a rechargeable occasionally (completely discharging it before charging) will prevent the development of "memory", which is a condition in which a battery may be fully charged, but will not discharge beyond a certain level, the result being that it doesn't last as long as you would expect.
This was an issue with NiCads, but I'm not too sure about NiMhs- some makers claim "no memory problems" for their NiMhs, but as I say, I'm not sure. Here's a link to a page that discusses battery memory, and if you google, you can find plenty of info:
My charger has the capability of cycling the batteries, but I usually forget how many times I've charged a set (it's suggested to cycle them every 10 charges), and just don't do it. I guess you could keep a log, but that seems like a lot of trouble.