I remember our interpreting instructors telling us that for some hearing people, mouthing just becomes part of their "hearing accent" and many if not most hearies mouth to some extent. Ours NEVER did during quizzes and tests, though. We got pure ASL.
Personally I know I mouth to some extent when I'm teaching my students, but since I'm teaching English I don't worry too much about it. If I'm only doing a lecture in ASL then I don't as much, but I don't do a lot of lecturing. The students do it as well, especially when asking a question and using SEE to represent the exact words of an English sentence.