If it's my first time watching a movie, I prefer to not use subtitles. Any time after that, I like to have subtitles, regardless of how well I feel I can understand their dialogue. I don't like to miss anything that they're saying, and I can read subtitles pretty well without being distracted (I'm a very fast reader), so unless it's my first time watching the movie, I like to have them on.
I've never actually used subtitles for a TV show. Dialogue in TV is usually more focused on, and less obscured by SFX/music than it is in movies *cough* Dark Knight Rises *cough* so I don't think they're really necessary for TV.