There are a few cases where I do:
1. When a guy that I hardly know shows interest in me and compliments.
2. When a friend starts listing all of the good qualities that I have and they don't (it depends on the friend though, I don't usually feed into the skinny girl who complains about being fat just so that someone can say, "no, you're not fat. you've got a great body, I wish I looked like you, etc etc.")