I’m surprised that no one else has answered this one yet… so let me pass on our experience.
Yes, because we publish incrementally, (either a single book or set of books each year) we have a separate project that we copy those “finished” books into once they have been typeset and sent to the printer. Our main reason for doing so is that we have been doing the audio recordings (using HearThis) of these books to go along with the Android App. Meanwhile the “main project” continues to be refined - with key terms, spelling and other improvements (which often mean that the latest living text is then out of sync with the audio). We never edit that copy of the project. It is a static snapshot of what went to press.
And yes, when publishing partial books (like GEN or PSA) we can copy across the whole book and delete what isn’t yet checked and approved. It gets a bit tricky later when you have to just add new chapters to these partially completed books (because you don’t really want to copy the whole book over - instead you want just the new chapters).
And now as we complete the NT, it may be helpful for the final revision consultant to see what all has changed in the live text since these books were approved for publication several years ago (by comparing the static copy with the current/latest copy of the project). Of course, you could also do this with project history, but then you would have to know exactly which point in history was relevant for each book)
So, I don’t know what the “best practice” is, but this certainly has worked well for us.