Keep the Mac up-to-date. There have been several battery software updates that maximize the performance and service life of the battery.
The Macbook has a lithium polymer battery pack that attains maximum service life if the depth of discharge is kept light. Say, use 30% of the available charge, then recharge. Li-polymer cells' lives are shortened by repeated deep discharge. Luckily, the Mackbook will not allow the battery pack to discharge to injurious (to the battery) levels. The Mackbook has temperature, current, and delta voltage sensing that prevent it from over-charging the battery.
It sounds like your two friends are partially right, but neither applies to the Macbook. In the old days, deep discharging of nickel-cadmium batteries was recommended to equalize cell voltages and prevent polarity reversal across weak cells during recharge (which would short the weak cell and ruin the battery pack). It is true a lithium cell will eventually be ruined by out-gassing hydrogen if constantly charged - but a Macbook's charger will cut off charging before cell damage.
So, leave your Macbook plugged in at your desk, so it has a full charge when you're ready to go mobile. Also note that a lithium cell can lose up to 5% of its charge per day, so staying plugged in keeps your run time topped off. Qualitatively, my Macbook seems to lose well less than 5% per day when unplugged.
Bill
