its hard to actually tell what the NVC actually wants!
i have seen a few posts where people have said to make sure to scan back and front for various documents though and this NVC website - https://travel.state.gov/content/travel/en/us-visas/immigrate/the-immigrant-visa-process/step-8-scan-collected-documents.html says to 'Include the front and back side of any document that has stamps, seals, or writing on the back.' but who knows if that is the actual reason for rejection. usually the rejection reason seems to be a generic one.
in terms of new uploads its a risk. some people say it puts you to back of line others say if you load under additional document it doesn't affect your position, no one really knows for sure how NVC works! We uploaded all our docs on 19th may. then uploaded some more docs in june so will see if our documents get reviewed over next 2 weeks or if the new docs pushed us back (i hope not!)