Hi dc,
I recommend you turn OFF (charge) when you are not using it because it'll save you $$$ on your electric bill, and because the tube stage and amp stage don't need that long to warm up. There is a soft start circuit for the tube stage and amp stage, so turning on/off puts minimal stress on the components.
But if you prefer to leave it on all the time in AC mode, it will be fine.
Vinnie