There is not much of a direct harm that can cause to your laptop if you charge or plug-in always though what can cause damage is increased temperature. When you plug in the laptop for all day, it might increase the temperature of the laptop and eventually shorten the battery life.
So, in case you notice that the temperature of your battery have heated up quite a much, you can put it in a water-tight bag, in a fridge, not a freezer, a fridge for a while. It will run it down to 40% of the temperature. Also if you are using, Lithium-ion or Li-ion battery, it should not be frequently discharged fully and recharged, so you should charge them regularly.