I didn't try pointing the camera elsewhere - I'll run that test when I have a moment. I usually make off-stage stuff invisible unless it's supposed to contribute shadows or reflections, but I don't know for sure if there's an advantage.

According to my test, in rendering terms there's nothing to be gained by deleting unseen items as opposed to making them invisible. Looking at the bigger picture, hidden items will contribute to other factors like file size, loading time and memory use, of course. I usually hide things until I'm sure I no longer want them in the scene, then I delete them. Then I discover I did want them in the scene after all. :)