Using a MacBook day in and day out, should it always be left plugged in?

Batteries - Maximizing Performance - Apple

There is nothing we can do to influence a Mac's charging algorithms. We can neither improve upon it nor make anything worse. Use it the same way you would use an iPhone. Charge it when it's convenient, or required, or whenever the mood arises.

It is neither good nor bad. Do as you please. Obsessing about a Mac's battery in the manner you describe sounds like a lot of effort to me though.

A portable device wouldn't be very useful if it were always tethered to a power outlet.

Plug it in whenever it's convenient to do so, when needed, or whatever.

Battery depreciation costs about 50¢ a week. I understand that might be expensive for some people, but in terms of the overall cost of Mac ownership, it's a difficult claim to justify.