DO NOT let your Lithium Ion battery drain to zero. This will only deteriorate the battery's life cycle. Lithium Ion batteries should never be "deep cycled." This is a concept left over from old Nickel Cadmium batteries, but does not apply to Lithium Ion batteries. Draining your LiIon battery to 0 before you recharge to preserve the battery's life cycle is a myth. Read more about the chemistry behind the two battery technologies here:
http://www.batteryuniversity.com/parttwo-34.htm
" Avoid frequent full discharges because this puts additional strain on the battery. Several partial discharges with frequent recharges are better for lithium-ion than one deep one. Recharging a partially charged lithium-ion does not cause harm because there is no memory. (In this respect, lithium-ion differs from nickel-based batteries.)"
...And there's more from wikipedia:
"Unlike Ni-Cd batteries, lithium-ion batteries should be charged early and often. However, if they are not used for a long time, they should be brought to a charge level of around 40% - 60%. Lithium-ion batteries should never be "deep-cycled" like Ni-Cd batteries.[7]
Lithium-ion batteries should never be depleted to below their minimum voltage, 2.4v to 3.0v."
...And finally, from Apple's own website:
"Lithium-ion batteries pack in a higher power density than nickel-based batteries. This gives you a longer battery life in a lighter package, as lithium is the lightest metal. You can also recharge a lithium-ion battery whenever convenient, without the full charge or discharge cycle necessary to keep nickel-based batteries at peak performance. (Over time, crystals build up in nickel-based batteries and prevent you from charging them completely, necessitating an inconvenient full discharge.)"
http://www.apple.com/batteries/