I have done both. When I record a stage performance, I will take the audio directly from the mixing desk to my Marantz PMD-660. I sync this up with my Sony Z1U camera footage in Vegas and I get about 20 minutes before I have to split the external audio track and resync it; usually just by a frame or two.
I have also added CD music to a performance after the fact where there was only a piano accompaniment that night. Since the pianist was not listening to the CD tempo, I usually have to cut and sync every measure of the song to get them right but the addition of the full orchestra from the CD increases the production value so much that it is worth the effort.
~jr
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