Just how many "generations" of rendering are you considering?
In any event, if you ARE worried, why not just top and tail with the unchanged media? I would and do. Now if you are considering re-rendering MPG or something other than uncompressed DV values, then you will introduce a VISIBLE reduction in quality. But you knew that?
Or are you doing this for some "other" reason you have not as yet told us?
So, stop worrying and just bring in your stock top-N-tail footage each time, and be happy.
Grazie