I leave mine on when I am in the room, but turn it off every time I leave the room. The same power point is connected to a lamp as well, so I just turn off at the power point then I know it is off and safe if the lamp is off. It's a very old iron so I am pretty careful with it.
I don't know how this is effecting my power bills yet, but I only have it on when I am using it a lot. Once i am done with piecing it is rarely on unless I need it.