All laptop batteries have their own under/overvolt protection circuitry, if the cells are charged they're charged - having it plugged in will make no difference as the protection circuit isolates the cells. In addition to that, lithium cells have virtually no self discharge so once charged they stay charged, they don't gradually drop below the cutoff and charge again (repeating the cycle).
The above means that the cells don't really go through any charge cycle, thus not introducing wear.
However, capacity does suffer slightly with lithium cells when being fully charged, but this effect is quite insignificant imo, the quality of the actual battery completely over shadows this effect. For example my work sony laptop battery was still going strong when I left and that was over 5 years old and that was plugged in at my desk 95% of the time, where as my home cheapo acer laptop battery made it just over 1 year being treated much better.
I would say not to worry about it, unless you know your not going to need the battery for an extended period of time (like +6 months) then it is best to take it out but make sure you are storing it at around 50% charge.