That's only true if you're updating your assets faster than your cache TTL. I suppose if you're a continuous deployment shop that deploys to prod 25 times a day, that's a concern, but not if you deploy once a month.
Cache granularity absolutely matters regardless of your cache TTL, because as I mentioned in my other reply in this thread, by bundling rarely accessed and frequently accessed resources together you are pushing other frequently accessed resources out of the cache.