When writing music it seems perfectly natural to start at bar 1. But when writing music to picture, I prefer to have 4 empty bars at the beginning and start the music at bar 5 for a few reasons.
One reason is technical. Most of us write using a DAW (Logic, Digital Performer, Cubase, ProTools, Ableton Live, Reaper, etc.) and then export a MIDI file that is subsequently imported into our favorite notation software (Finale, Sibelius, Dorico, etc.) for orchestration as well as into ProTools for live recording. As we go from cue to cue we write automation to our MIDI controllers, and the controllers stay where they last were when going back to the top or to a new file. By starting at bar 5 you can write controller data in bar 1 that resets all your controllers.
Some DAWs (like Digital Performer) allow for zero and negative bars, which can be used for that purpose. But if we use a zero bar or negative bars it complicates matters down the road. MIDI files don’t understand negative bars, and so when we export a sequence that starts at bar zero for example, the MIDI file will see bar zero as bar 1 and all of our bar numbers will be off by one. We could manually make the adjustments after importing into our other software, renumbering the bars to fix this offset, but if the music starts at bar 5 and we never use bar zero or negative bars to begin with that’s a non-issue. It keeps things a little simpler and minimizes the potential for mistakes.
Another reason is that it’s not uncommon to get notes while writing that require us to start the cue a little earlier “can you sneak the cue in before that moment rather than coming in right on the moment?” If our music started at bar 1, we now have to insert bars at the top of our sequence moving everything. If our music started at bar 5, we have some empty bars and we can simply start the cue at bar 3 or 4 or whatever works without having to change anything else. This may not sound like a big deal, but when you’re under a tight deadline every little thing adds up. And if a cue already went out for orchestration or prep and then you had to make the change, it’s a simpler and faster change to make everywhere than adding and renumbering bars would be.
There are other reasons for starting at bar 5 instead of bar 1, too. Historically we needed a decent amount of pre-roll for the recorders to sync up, and having those 4 empty bars was usually enough. This isn’t really an issue these days, but I’ve been doing this long enough to remember when it was.
I generally find that starting at bar 5 (which has become pretty standard in film/TV scoring) keeps things a little simpler, which means less room for errors and less time spent on making adjustments.
If you have any questions, feel free to contact me.