This question ("plugged in all the time or not") always brings up strong discussion. Quick search through here would show ~weekly fights.
There seems to be more or less consensus (among experts who have education/expertise as well as Apple) that keeping battery always plugged in is not good for battery and will result in premature death. The other extreme - running it through lots of cycles is also not ideal - typical lifetime is ~1000 cycles (and by then the capacity Apple guarantees is 80%).
Note, that time is also battery killer. Even if nearly unused, old batteries die. Some reported receiving old battery (old manufacturer date on it) and it died or swell (which is even worse) very quickly if it worked at all. Apple knows this and typically will install fresh batteries - my replacement is manufactured August 1 and it was installed into my MBP mid September.
So what to do? YOu will get lots of contradictory advise... I personally discharge my battery to 40-60% every few days intentionally (usually at home using my 4k external monitor, uses quite a bit of power to drive that). Most of my use is on charger. After 6 months of use (since replacement due to keyboard replacement) I have 42 cycles on and 99% capacity.
Do not get too excited by the exact capacity value, mine varies between 97 and 100% the last few months.
