I group everything into buses: vocals to a vocal bus, guitars to a guitar bus, bass to a bass bus (even if there is only one bass track), drums to drum track, etc.
The reason: I automate the volume on the individual tracks. Then if I decdie, for example, that I need more or less strings, or need the string section to fade out, I can do that with the bus instead of messing up all the volume automation on all the individual string tracks. I may have the mix finished, but decided I need more drums, or less drums.
Another use: when a singer is recording, I can pull the instrument buses down so she can hear herslef without messing up my mix. That's one reason the bass has a bus even if it's just one track and doesn't have a lot of automation.