You should never let it run dead. That kills LithiumIon batteries.
You can run it on AC power mostly, but should let it run down to 40% (some say no less then 50%) at least once a month and then recharge, just to exercise the battery.
Remember MBP batteries have a cycle count of 1000. 1 charge from 1% to 100% is one cycle, 2 charges from 50% to 100% is 1 cycle, etc.