My 2 cents (which probably aren't worth that much) on this topic:
In regards to the original post - having read countless posts and articles about the mastering process, especially in the DUC forums, supposedly there is a difference between master processing on the output bus of a full multi-track session and doing the work on a bounced/rendered 2-track file. The difference is related to how DAW sum on the master bus, and how engineers/trained ears perceive the difference.
To decide what YOU should do - I would say: A/B both versions - 1 coming off the summed multi-track, versus 1 coming off a rendered 2-track. Both with no output bus processing. If you can hear the difference, then make it a two step process. If you can't hear a difference, then 1 step it is.
If you have plans to send out your track to a mastering house, don't put anything on the output bus. That's just annoying to the mastering engineer (the ME). Let him do his work - he's getting paid enough for it.
If you have plans to release the song with other songs on a medium, don't put anything on the output bus. The ME (even if its you) needs to master the songs with one another, finding common ground between each, and making a finished singular project.
Having said all that, I do all my work with a big fat mastering suite on my output bus. You may listen to my stuff, and think it all sounds like ass, but, to me, its fine, and since adopting this practice two years ago, I've not gotten one complaint concerning my sound quality on projects I've sold. I'm not interested in training my ears anymore - I spent years doing that while I was a recording engineer. I could give two shits if I don't remember how to differentiate between 1000 Hz versus 1200 Hz. My job is to make good music, that sounds polished enough so that the consumer never even thinks about the quality.
So, while I work, I keep that mastering suite on my output bus, and I compose, mix, process, all at the same time.
Having said that, I learned and spent years breaking that process into each of its tedious-ass steps: composition, rough recordings, drafts, tracking, eq-ing, creating balancing the stereo-image, additionalproduction/processing, auditioning on different monitoring systems (still do this plenty before finalizing the master), bouncing the multitrack, and then sending out to mastering.
Can some mastering engineers do black magic on my tracks to make them sound better? Yes. Is the cost worth it at this point? No.
DJ Franco de Leon
www.FRANCODELEON.com