I would get a song as far along in Reason as I possibly could on my own. Especially with Reason 3. Then it's fairly easy to kill the effects and mastering with the flip of a a few switches in most cases, and the Mastering engineer can work with a "virgin" copy plus he can get a sense of what you are hearing by listening to your "mastered" version.

The approach I take with Reason is to try and use it where it is strongest and to try and not rely on it too much to do the things that it does with less convincing results.

For example, I really appreciate the great sound quality and diversity in the Orchester sound bank and it's really fun to create that rich orchestral sounding music, but, I know it doesn't compare with the real thing. Not even close! So, I wouldn't really try to emulate that live, acoustically produced kind of sound for anything I was seriously trying to produce.

I hear those kind of tracks and even if the musical compositon is brilliant, it just doesn't come across as pure as a live human performance of the same piece of music does.

However, when I hear a piece of "experimental" or "electronic" music, or something that is clearly not pretending to be the real deal, I find my ear can be much less critical and I am more lenient about the sound qualities of those type of patches and samples. Then those Orchester cellos, string sections, brass and those full on symphonic Combimator patches becomes a vast and powerful source of sounds.

So, in that sense I feel Reason is one of the best means ever available to produce incredibaly rich, vibrant and beautiful musical masterpieces within its own domain.