Should I leave my Macbook plugged in when using it?

Leave it plugged in whenever convenient to do so. Neither your battery nor your computer will be damaged by leaving it plugged in. Leaving it plugged in reduces the rate of accumulation of battery cycles.

Yes, it is quite safe and recommended to leave it plugged in.

When near an AC outlet, leave it connected to AC power.

When you need portability, use it on battery power.