Should I keep my MacBook plugged in constantly?

+1 to the guidance provided by our friend, @Kilgore-Trout. 👍🏽

Keeping the laptop plugged in all the time should cause no problems.

Additionally, here is some of Apple's guidance regarding battery performance, health and management:

Batteries - Maximizing Performance - Apple Support

About battery health management in Mac laptops - Apple Support 

In a nutshell, to get the most from your battery:

• Let the OS and charging circuits manage the charging.

• Plug in when using the computer near available power.

• Use on battery when ac power is unavailable or when you must be mobile.

• Don’t subject the computer and battery to extreme temperatures.

• Don’t allow the battery to regularly discharge deeply before plugging in. Yes, it may happen from time to time, just don't make it a habit.

• Don’t obsess over battery life and/or health.  

Should you? Odd question. Can you? Yes. However, make sure you are using battery optimization setting. And its not a bad idea to uplug it every once in a while and cycle through a battery discharge cycle.