In re-reading Ansel Adams' "Personal Credo", published in 1982, I came across many concepts that absolutely apply to photographers today. Perhaps the most interesting was his take on image manipulation.
Although he is referring to the wet darkroom, there is a clear parallel to the modern digital darkroom.
"As long as the final result is photographic, it is entirely justified; but when a photograph has the "feel" of an etching or a lithograph, or of any other graphic medium, it is as questionable as a painting that is photographic in character."
In addition to this piece from Adams, I also found a terrific Nat Geo interview with photographer Jeff Hogan. When asked "How much fiddling with images is okay, and what goes too far?" Hogan responded..."Again, it depends on your audience and your goal, and what you are claiming to be doing. I think it's acceptable to clean photos up, adjust contrast and color, that sort of thing—but I've seen people do things like add more animals to a zebra herd to fill it out. That's misleading; it gives viewers a false sense of that environment, of how plentiful a species is. Now, if that's natural—to see bigger herds [like the one "created" in the image]—then the filmmaker should just wait and film the real thing rather than faking it." Read the whole interview here.