Every time we unplug the power, the Mac will start to discharge the battery. In this manner, the battery lifespan will shorten.
The answer would be to leave the charger on whenever possible, as the charger actually stops charging the battery once it is full.
Check this on your Mac: Click on Apple Icon > About This Mac > System Report > Power
See the "Health Information:" for cycle count and the following:
AC Charger Information:
Here's an article discussing it: Practical Tips For Taking Care of Your Mac's Battery
I remember having read an article by Apply saying not to let the battery totally drain, as it will affect the battery lifespan. It's best to keep it around 50% and start charging whenever possible. This applies when not using the laptop for a long time - occasionally bring it out and charge it.