Any copy that's been synched to video will have undergone lossy encoding again, so the general fidelity is lower than the LD's 384 kbps ac3 on its own. It still has a good sound, but you definitely wouldn't want to base any further editing on that.
I got away with using the imperfect captures for the 70mm, because the LFE channel was the only part of the '97 mix I actually worked with, and most of that is silent anyway, so any errors pass by unnoticed.