If it's done well, I shouldn't even be able to notice it, and it should make sense as part of the story. Only when mishandled does it turn the narrative to shite.

Excellent point. And now that I've received so many thoughtful responses, it does appear that there's not a hard and fast rule, and even if there were, someone would be bound to break it and still win awards, so… there you go.

For me, my personal preference is "pick one and stick with it" but that's just me. Thank you for writing! And your comments are both thorough *and* succinct. :)