The battery is most under stress above 90% and under 5%. It also has a limited number of charging cycles, and its health decreases with each one. That being said, the lower the discharge depth, i.e. the difference in stored capacity between when you unplug it and plug it back in, the lower the damage.
According to this website, you would get the longest battery life by always keeping it between 80% and 40%. If your laptop comes with a battery utility that allows you to set charging thresholds, you could stop charging somewhere between 60% and 80%, thus keeping your battery out of the stress zone. In that case, leaving it plugged in wouldn't do almost any damage.
If you don't have access to such a utility, it's probably better to unplug your computer at about 90%, then plug it back it at about 20%, rather than leaving it always plugged it. Of course this is impractical and could become annoying, and you should keep in mind your battery will degrade either way. So the best solution might actually be to use it anyway you want, and should it fail you before you buy your next laptop, just get a new battery, which is usually quite cheap on amazon or ebay.