Leaving your laptop plugged in all the time is perfectly fine and keeps your laptops battery fully charged and ready to go.
I always keep my laptop plugged in when I'm at home or near an electrical outlet to keep the battery in the laptop charged up and it's never been a problem.
The laptop contains a circuit that shuts off the charging voltage to the battery when the battery is fully charged to prevent overcharging so it's safe.
Even leaving cell phones plugged into the charger all the time is safe because there's no way to overcharge the cellphone as it will automatically stop charging when the battery is fully charged.