I'm also with Jon on this one. I have never had more than a pair of batteries with any camera. When I acquire a grip, which I have for all but one, I top the grip battery off prior to shooting. At the time that the body battery depletes to 50%, I charge both and swap their positions.
One battery lasts me so long that it wasn't until I bought a grip, after the camera was 90 days old, that I saw the one battery was showing as age 3. If it doesn't start reading as new after a few full discharge/recharge cycles, it will be replaced.
Scott Chapin Powder Springs, GA, USA Nikonians Team Member