I haven't made any of my own yet from scratch, so I'm definitely no expert or anything. But I remember reading this about ingredients, breathing, steeping, etc.:
1. If your flavorings had any form of alcohol as an ingredient...you
may want to let it breathe so it would evaporate off.
2. If a recipe had
only fruit flavorings in it, that it should be OK to use once mixed well, because it wouldn't have to be steeped.
3. Most other recipes/ingredients will definitely do much better if you let them steep. But that's not just putting them somewhere cool & dark and leaving them alone. Regularly shaking them every now and then during the steeping time is recommended, too.
Would probably be best to find someone's post where they list ALL those blogs people have written about steeping, breathing, etc. and read
through them to be sure. I plan on doing that before mixing mine.