I think it is okay to change a person's appearance, in the situation of a model agency.
Although it isn't okay to do it in photojournalism.
Cropping, fixing scratches, white spots, dust specs are all okay.
The difference of fashion photography and photojournalism is that fashion photography is okay to change the appearance because your trying to promote the clothing rather then telling a story. It's a BIG NO NO to do so in photojournalism because your supposed to tell the truth, and not manipulate a person's apperance or anything but actual photo defects.
In fashion it has teenage girls have the wrong picture of true beauty, because when you see people in magazine's their fake and put a distorted image of the real person. It doesn't effect ethics too much.
On the other hand photojournalism is SUPPOSED to tell true stories, and actual reality, and ethics is a big contraversy because, there are many situations in which it would be okay for one thing but wrong for another.