i pretty much concur with much of what has been said. it depends on the goal and context.
for news reporting, for comparing camera makes, then alterations are not acceptable.
but when it comes to artistic photos of any kind, then anything goes. it is up to the artist and his vision. that is his only responsibility, to his own vision. if he wants to stay true to the scene, that is his right. if he wants to move around elements, change colors, hdr, layers, etc, that is also his right. it is then up to the viewer to decide if they enjoy that vision or not.