Based on a sample of one, I fall in the "leave 'em discharged" school of thought. About 12-13 years ago, I bought a DeWalt 18V drill driver that came with two NiCad batteries. For no particular reason I used and recharged the same battery for about ten years, until it wouldn't hold a charge long enough to run a couple of screws.
I had gone so far as to mark the one I was using so I could "save" the other one. When I charged the previously-uncharged batter I figured it would be bad just because of age. Here I am about three years later still using my "saved" 13-year old battery. My drill driver receives occasional homeowner use. I charge the battery either when I know I'm going to be away from the charger to drive a lot of screws, or when the battery gets weak.
