Is good or bad practice to leave charging after phone is fully charged ?

As mentioned, you cannot “leave it charging”. It will only actually charge the batter when needed. The power brick is not a charger, it’s just a power supply. The device itself regulates its battery charging. And like all industry standard S.M.A.R.T. Lithium batteries, those in Apple devices cannot be over charged.

Yes. You cannot over charge the iPhone. The iPhone will stop charging once it reaches 100%.

Charge whenever and for however long it is convenient to do so.