Music is like color or acting or whatever. It's really something I think about from the beginning. Not that I always know exactly what I'm going to use, but I don't see it as something like, "Let's find some songs now!" after we have a finished film.
I'm at a stage in my career where I don't expect or get too much editorial input into what I'm doing. I have a proven track record of success, so my editors are willing to cut me some slack even when a particular approach is not to their personal taste.