Well, it depends on what the final product will be. The only full 5.1 track that I created was for my prototype DVD. I did experiment with the Jaws "da-dum" music, rotating the 5 channels in a circle around the listener. THAT was an awesome effect.

For the most part, I usually try to keep the audio in PCM, unless I need the disk space, but a 2.0 track can be made straight from the stereo track off the timeline. A 5.1 track, done correctly, requires 5 separate streams, each designated for a specific speaker.

What's nice about Vegas is, if you are creating a new video edit, and want to create a new 5.1 soundtrack to go with it, you can run the AC3 through Besweet, get your five streams, place them in the timeline under the video, and link the video to all the audio streams. Whenever you make a cut and move video, all the streams stay together, and your final audio will match the video.