There is never going to be a case where the world misses the answer to the ultimate question of life, the universe, and everything because it was said by a Nazi and everyone refused to listen to the Nazi. When it's clearly straight up propaganda, it's perfectly rational to dismiss it due to the source and not investigate further. If there's a valid and useful point to be made, it'll get made in more respectable sources too and then it might be time to pay attention. Plus, even if they do cite sources, it's hard to spot where they've twisted or lied about those sources, but it's really easy for the propagandist to spout whatever nonsense they believe because they don't care about the truth. That asymmetry is good for the Nazi and bad for decent people, and the way to fix that is don't waste your time carefully investigating and critiquing Nazi bullshit.