Leaving it plugged in does not cause any problems with charging. In fact, it is required that lithium-ion chargers stop before reaching even the slightest amount of overcharge, because overcharge is actually a serious safety hazard. It is simply not allowed.
Just don't let it empty fully on a frequent basis. That is bad for it. It's much better if it's topped off after you drain it part way, instead of letting it continue to empty.
The only risk to the battery when plugged in is if the machine is running hot for a long period of time (because it's working hard in a game or appilcation). High heat will shorten the life of a fully charged lithium battery. I'll usually unplug and let the charge drop a bit if it's hot.
But like the others say, it's not worth your time to micromanage the battery too much. Use it the way you need to use it.
Supporting information:
How to Prolong Lithium-Based Batteries