I'd keep it plugged in, except if not in use for long periods.
Some Lithium Ion batteries can get bloated if overcharged. The specifics of exactly how to reproduce the problem are unclear to me, but it does happen, as i've had this happen to me in the past. It used to be a common occurrence on some Macbook models. So be careful if you leave your laptop unused for long timesperiods with the battery inside and connected to power. This happens due to release of gases (hydrogen) inside the battery package which will make it swell visibly. It will make the bloated battery pop out, since it will no longer fit into the battery compartment, and will make the battery unsafe to use due to the danger of combustion.