never plug either of mine in (have to cycle the key once in a while)Dont see why you couldnt leave them plugged in 24/7, its a low wattage heater so no boiling should occur.
If you did plug one in the only reson I can see for unplugging it would be save on the electric bill and to prevent pulling the socket out of the wall when you drive away which for me is teh biggest reason not to plug one in,