I won't keep it connected 24x7 if the power LED on the charger is on, even when your phone is not connected.
I personally prefer it turned it off when I don't need, and my car has a dedicated toggle switch (diy project) to turn it on/off.
however my charger is installed in console instead of lighter socket.
if you want you can get switch installed for lighter socket, it won't cost a bomb, could be done under Rs.100/-