Nope.
When I first got the RFE requesting the proof of income, I also received a message on CEAC saying that a joint sponsor could be added to avoid any delays (since I didn´t meet the income requirements). At the time, the NVC call center was still working so I called them and was told that the joint sponsor paperwork can be taken directly to the embassy at the time of interview. I was also told that the NVC doesnt even look at the proof of assets, that´s done by the consular officer at the time of the interview.
In summary, I chose not add a joint sponsor when I got the RFE because it would delay the submission of the proof of income documents (that I already had in hand) and could potentially generate another RFE for problems with the sponsor documentation.
In any case, as soon as I get an interview date, I will put together a joint sponsor packet and my wife will take the physical documentation to the consulate in case they request it.