See the following webpage:
http://www.batteryuniversity.com/partone-16.htm
On this page, it says:
Manufacturers rate the lithium-ion battery at an 80% depth of discharge. Repeated full (100%) discharges would lower the specified cycle count. It is therefore recommended to charge lithium-ion more often rather than letting it discharge down too low. Periodic full discharges are not needed because lithium-ion is not affected by memory.
(This website, BTW, is an excellent resource for all kinds of battery information.)