The battery include a so-called "fuel gauge" chip that keeps track of the charge level, health status and individual cell balance. This chips is providing the battery level measurement that is displayed by OS X. As a result, you may install a new battery without consideration of its current charge level -- whatever the charge level actually is, it will be correctly reported by the battery chip to the OS.
As for all LiPo batteries, the main recommendation is to avoid regular deep discharges. There is no memory effect -- deep discharges degrade the chemistry and reduce the battery life, so try to keep your computer plugged as often as practical. An exception to this rule would be long-term storage of the computer. In this case, a charge level of about 50% is preferred (as per Apple recommendations).