Either is acceptable -- well, I take that back. You either want to attach it to the reverse lamp 12volt supply, or the accessory 12volt. Don't wire it to a constant 12volt supply, as it will drain your battery when the car is parked.
As to which to do, it depends on when you want to use the camera:
- If you want to only see the rear view camera when backing up, or if your head unit switches automatically to the rear view camera when the camera is powered on, then you want to attach it to the reverse lamp.
- If you want to be able to use the rear view camera while driving and when backing up, and you can switch to the rear view camera manually (i.e. button on the head unit), then power it to the accessory 12 volt source. Note that this way will not automatically switch to the rear view.