It depends on the context, of course, but I would hold photojournalists to a pretty high standard - no added objects, no merging of scenes, etc. Really anything other than correcting artifacts of the photographic process should be taboo. This is not a new issue, but goes back at least as far as photojournalism. There are allegations that Mathew Brady reposed corpses for some of his more famous shots. On a more mundane note, when my mother worked for the "Women's" section of the local paper back in the 70s, her editor insisted that all cleavage be airbrushed out of any published photos.
For art, I think anything goes, but there is some degree of disclosure that should be performed.