The most expensive moment to start on the music is when the edit is locked. Unfortunately it's also the most common one.
I understand how it happens. The shoot is done, the edit takes longer than planned, and music feels like something you can add at the end. Like the colour grade. So the composer gets a finished film and a deadline, and is asked to make it work.
Why that costs you
Once the edit is locked, every musical choice becomes a repair job. The tempo has to fit cuts that are already there. The build-up has to land on a product shot that's already placed. If the cuts fall every 1.7 seconds and no sensible tempo hits them, someone has to cheat, and you can usually hear where.
It can be done, and we do it all the time. But it almost always ends in a compromise somewhere.
What changes if you start earlier
At the rough cut, everything is still loose. A cut can move half a second to land on the music instead of the other way round. That costs the editor nothing and you can actually hear the difference.
On the Boska campaign we were in from the early cuts, and the music changed along with the edit as it got tighter. So when the picture was locked, the music already fitted it, and we could spend the time making it better instead of making it fit.
You also find out early whether the tempo works at all. Send us a rough cut and we can usually tell you within a day.
What you need to send
Not much. A rough cut, even an ugly one. A sense of the lengths you'll need (the 30, the 15, the 6). And a couple of reference tracks, ideally with a line about what you like in them.
That's enough to start. Nothing has to be final, that's the whole point.
So next time you're planning a shoot, put the music on the schedule next to the edit rather than after it. It's the same budget, just spent at a moment where it does more.