There is no harm in leaving it plugged in while the car is off. The power to the outlet is probably cut off when the key is removed from the ignition. Leaving it plugged in might lead to a miniscule amount of fuel use, but it isn’t worth worrying about. In fact, it is a good way to keep track of the charger. It is always in the right place when your partner looks for it. There are more important things in life to worry about, like which way to hang the toilet paper roll.