Thomas, that's a good question. I have found that if my goal is to make the sitter look as beautiful / handsome as possible, I can almost always produce something that is both honest and appealing to the subject -- they're nearly always excited about the way they look in the portraits. However, there ARE occasionally people (who am I kidding -- they're always women) who cannot accept their appearance no matter what. I could photoshop them into oblivion and they would still hate the images because their self-image is so warped. Really, though, that's a rarity for me.
Now, on the other hand, there are times when I'm not trying to create a flattering portrait. Sometimes it's about emphasizing character or age. The Regulars series that I did documenting the people at my local tavern is a good example of that. Those images, I generally don't even show the sitter. If they're not commissioned, I am really not under any obligation to share them.