I've worked on several Broadway shows, and it's VERY rare for music to go straight from a composer to an orchestrator. The reason? The composer is not a part of the day to day rehearsal process, and often doesn't know exactly what's happening with his song. Here's the procedure as I know it:
The composer writes his song...
The arranger takes the song, and it goes through the rehearsal process, with music added by the arranger. Here are the some the things the arranger is likely to change:
The intro, depending on how the song fits in the show.
The key, so it fits the performer's range.
Vocal harmonies, if there is more than one person involved.
Dance music, if this is part of the number.
Music under dialogue, if there is dialogue during the number.
The finish, so that the number gets a big hand.
When the composer, director and choreographer decide a number is ready, the arranger will write a chart showing all of these changes, and the orchestrator will come in and watch the number. Then, everybody will go into a room and tell the orchestrator what they feel he needs to know to make the right choices. Then the orchestrator takes the arranger's chart home.
I'm sure there are exceptions, based on the composer, and the type of musical, but this is how it's always worked with me. Often the composer will step in and ask for changes during this time. But the composer's music is usually only a starting point.
Behind the fake tinsel of Broadway is real tinsel.