See the Apple article below.
It is OK to leave the computer plugged in, but be certain to disconnect and use on battery power a couple times during the week, or when you need portability.
Nothing to worry about, really.
I leave my MacBook Pro plugged into mains about 98% of the time (mainly because I have a 27" monitor and use it primarily as a 'desktop'). But I have a reminder setup for every two weeks and then I will disconnect from mains and let the battery run down to 30-40% and then connect to mains again. This keeps the ions moving and the battery healthy.
I bought my current MacBook Pro in March of 2012 and my battery 'health' reads between 98-99%.
It's a good habit to simply disconnect from mains once every couple of weeks and let teh battery run down to 30-40% before plugging her back up.