In previous hardware releases, it wasn't a good idea to leave a MacBook "plugged into the charger all the time", because this could lead to a condition where the battery starts to expand -- a fire hazard.
The latest MacBooks supposedly have software that will "automatically" prevent such over-charging, but... speaking only for myself... I don't really trust it.
My personal recommendation:
Don't leave the charger plugged in all the time.
At least once every 3 days or so, take it "off charge", and run from the battery alone.
Let it drop down to about 40% or so, then plug it back in.
My opinion only.