Since your processing post is Frankfurt, I’ll tell you my experiences and what I’ve also heard from others. The consulate didn’t take any of the supplemental evidence. All they wanted was the I-134 from my fiancé. I technically had one from my co-sponsor but it wasn’t needed. I wouldn’t count on your experience being the exact same, but I don’t think you need to worry too much. Tax documents and/or employer letters are probably better anyway. In the Frankfurt interview instructions it goes into what evidence to bring (even though they didn’t ask for any of it from me) - I’d follow that to be on the safe side