ethics in photography

When should one take liberty with a photo and digitally alter it to mean or say something other than what was intended? I think that this is a matter of personal preference. For me, I would never take the work of someone else and alter it with the intent of calling it my own. However, I do think that changing a photograph to fit the needs of a situation can be beneficial in certain instances. For example in the photo showing the effects of the depression on families, the photographer asked the children, who were laughing to turn around so that their faces would not be featured in the photo. While this may seem to be unethical, the truth is the depression was hard on families and to get that point across poignantly the photographer had to alter the composition of the picture. In the end, it is up to the artist what they will accept and will not.