A DP once said to me: Don't light with key, fill, and back simply because that's the traditional way - hell, that's a mainstay from the black and white days when we needed a backlight to seperate people from backgrounds. Do what's right for your story, sometimes we only need a single key, sometimes our backlight is our key. Stop thinking about light as a way to get an exposure, and start thinking about light as one of your main storytelling techniques as a cinematographer; it's one of, if not the only non-camera technique we have available to us as cinematographers so make sure you're using it to it's full extent.
So, now I sit with Directors when I come onto a project and we discuss the ideas and the themes behind each character and each scene, and I light the thing with all of that in mind. Sometimes that means under-exposing quite a lot, sometimes that means harsh lights with contrasty shadows, sometimes that means soft lighting, or faking day time.