Is better to keep a MacBook always charging or take off power adaptor as soon as fully charged?

You can leave the charger plugged in whenever it is convenient to do so. The battery and computer will not overcharge. There is no need to periodically discharge the battery.

The battery and its charging system in the computer are "smart." As archieny says, they are designed so a battery cannot be overcharged if left plugged in, yet it will be "topped off" if the charge drops below about 95%.

Some tips:

• • Don't routinely run the battery to zero charge. A couple of times won't hurt but doing it a lot will.
  • Ignore incorrect advice on third-party battery sites to "calibrate" your battery. Apple discontinued the need for calibration about 10 years ago. Calibration requires running the battery to zero--see my #1.
  • I've been here a long time and the batteries that seem to deliver the longest service life are those that get regular use. On the other hand, I wish I had a dollar for every post here that said,' Why did my battery fail so soon? I never use the battery!" I run my 2012 MBP on battery once a week because it fits my workflow, and the original battery still has 90% of its original capacity and very good runtime.

And my most important tip:

• • Don't obsess over the battery!

It'll charge it up to about 95%, switch to getting power solely through the power supply and use the battery as a "buffer" against surges so that the computer doesn't get damaged.