In my view, this is not a matter of ethics so much as it as matter of artistic judgment.
I can and often do manipulate images during the course of printing in my darkroom. Cropping, changes in contrast, local burning or dodging, spotting, even bleaching are fully acceptable tools to create a final print. And I don't have any problem doing the digital equivalent of any of those things when working with a digital file.
For the last few years, I've been the editor of an international technical journal that regularly publishes pictures from various technical conferences. These images are published to tell a story about some event. Often, the images are produced by amateurs whose photographic competence is far short of the average APUG member, so the quality is not always high. I don't have any problem with manipulating the digital files of those images to make them sharper, to crop them to better tell the intended story, and generally more publishable. I routinely clone out distracting highlights (I can't think of anything more distracting that a lighted wall sconce growing out of the left ear of an award recipient at a banquet), and I have no ethical qualms about editing 'grip and grin' type images to remove individuals (recognizable or not) who aren't actually a part of the story that the image is there to tell.
Where I do have a problem is where technology (whether it is digital or analog matters not) is used to create an image of something that actually didn't happen. Some of the more offensive images include a mountain landscape with a steam locomotive coming around a corner - supposedly from a place where there actually was no railroad. Yes, the image was pictorially nice - but it was a fabrication of something that was not real.
Years ago, shortly after the 1980 Winter Olympics in Lake Placid, a friend showed a print of a ski jumper coming off the end of the high jump at Mt Van Hoevenburg. And behind the jumper was a series of shadow jumpers, all lined up in a neat arc, suggesting that a number of jumpers had come down the jump together. In point of fact, the print was made by abstracting an image of the jumper using litho film, and then printing multiple images of the same jumper on the same print. Graphically neat, and clearly demonstrating great printing skill, but the final image was a total fabrication. It created an impression in the mind of the viewer that something had happened that existing only in the darkroom and the creative mind of the printer.
I find that kind of photography to be objectionable, regardless of whether it is done digitally or in a chemical darkroom.