How often should I use the battery on my MacBook (which I exclusively use as desktop)?
Taken from https://web.archive.org/web/20110521104819/http://www.apple.com/batteries/notebooks.html


  Standard Maintenance
  For proper maintenance of a lithium-based
  battery, it’s important to keep the electrons in it moving
  occasionally. Apple does not recommend leaving your portable plugged
  in all the time. An ideal use would be a commuter who uses her
  notebook on the train, then plugs it in at the office to charge. This
  keeps the battery juices flowing. If on the other hand, you use a
  desktop computer at work, and save a notebook for infrequent travel,
  Apple recommends charging and discharging its battery at least once
  per month. Need a reminder? Add an event to your desktop’s iCal. When
  your battery no longer holds sufficient charge to meet your needs, you
  may choose to replace it. If your notebook came with a built-in
  battery, you should have the battery replaced only by an Apple
  Authorized Service Provider.