Top comments in a thread frame discussion - if the top comment in a thread is a conspiracy theory, then it's likely that the entire discussion will be derailed. For that reason, I think it's valid to downvote such comments, to prevent them from derailing the discussion.
Doing so gets people to read both. Besides in the absence of the original comment (incorrect as it may be), the followups lose context.
Reserve down votes for opinions that do not deserve a reply. Such as pure trolling.