Note that unless you have changed your ingnition cylinder there is no ACC position. Only Off/Run/Start.
I wired my radio in the the IGN feed for the switched wire and the 12v feed for constant. After a couple days I decided to wire the switched side into a toggle switch so I could turn on the tunes with the truck off.
If you want to get creative you could run the switched feed from the IGN terminal to your 'switched' wire and a second constant 12v feed to it through a toggle.
Allowing it to function with the ignition with the toggle off, and always with the toggle on.
I don't think there would be any issues, but I would put a diode on the IGN feed wire just to be safe.
Just don't forget to put a fuse on it.