You've raised some interesting questions here. Replies will depend somewhat on your use of the audio being processed through the ACW and how the audio was recorded. Will the audio be used in your production or are you just determining the chords, progression and tempo to create a backing track? How much drift in timing will be more of a determining factor than whether the recording is commercial or a track you played.

Each of these scenario's use a bit of different approach than the other. If the audio processed through the ACW will not be used in the final production, the actual tempo map is irrelevant to the final production all though it may be used throughout the various stages of production before finalizing your song or cover. The average tempo determined by the ACW is all you need. If the audio is a commercial production, it has likely used either a click track or quantization has been applied in a DAW at some point and the variations you mentioned can be tightened up in the ACW prior to exporting the tempo map into BB.

If this is a current project you are working on or something that's been a frustration in past projects, give a little more detail and I'll be glad to share my approach to getting things to match up.

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