I agree that a lot depends on the movie, the sequence, the setting, etc. One of the best uses of music in a film, to me, was Spaulding Gray's
Swimming to Cambodia. The movie was basically a monologue with changing camera angles providing most of the motion. Laurie Anderson's score added depth and dimension to that film, yet never overpowered it. One of the worst uses of music in a movie, to me, was Lawrence Kasden's
Grand Canyon. There was a lot of music that seemed to portend a dramatic, traumatic event which never happened. I walked out of the theatre a nervous wreck.
In short, I guess I feel the music should touch the awareness, perhaps be "heard" when it is appropriate to the scene, and should be congruent with whatever is happening on the screen.
Of course, when you get to movies like Powaqqatsi, Anima Mundi, and other
Godfrey Reggio/Philip Glass collaborations, the music is as much a pert of the film as the image and needs to be heard.