One other thing I'll add to this discussion:
For fun I tried the following last night and it's (so far) helpful -- I added a third bus called "headphones" which is fed by the Master bus (temporarily). It has a headphone reference plugin (Waves Nx system) to use while tracking and mixing as needed. Before finalizing a track the setup is:
1. The Mixing bus feeds the Master bus and contains console emulations, multiband "glue" compressor (Ozone if necessary), stuff like that.
2. The Master bus feeds the soundcard but also has a prefader send to the Headphones bus, with the fader temporarily set at zero (to avoid sending double the signal to the soundcard). Plugs here include a Limiter, Sample Magic A/B for reference, SPAN. I was pleased to discovered these also work just fine with the fader set at zero (the Sonar singal flow chart was very helpful). Here's the key: I created a quick group for the mute buttons on the Master bus and Headphones bus, starting with one on and one off when the group was created. That way, when I'm ready, I mute the Headphone bus, bring the Master up to zero and it sends directly to the sound card.
This may all prove to be too complex in the end, but I like having the Nx system available and not in the Master chain. YMMV
Steve