Why downvote if it doesn't detract from the conversation?
My rule of thumb is to upvote if the comment is insightful or has information I find useful. Downvote if it seems blatantly incorrect, mean, overly negative or is actively trying to make the conversation worse. Otherwise I just ignore it.
Why do you feel that something that is incorrect (even blatantly) isn't useful? I have often observed a thread where somebody ignorantly spouts something and is taught appropriately. I've learned stuff from those and left with a net positive. The original poster might never admit to being wrong, or changing his world-view, but I certainly have many times.
In this case "blatantly incorrect" meant lies with a likely ulterior motive. Off the top of my head I can't think of any case where this has been an issue on HN, but I have seen it on other forums where marketers try to spin problems with their products.
My rule of thumb is to upvote if the comment is insightful or has information I find useful. Downvote if it seems blatantly incorrect, mean, overly negative or is actively trying to make the conversation worse. Otherwise I just ignore it.