EDIT: One thing I neglected to mention. You can probably tell based upon my comment being downvoted, you are not actually allowed to talk about downvoting. Its kind of like fight club.
And yeah, there is are guidelines specifically related to downvotes.
https://news.ycombinator.com/newsguidelines.html
If you're wading into contentious discussions, be prepared for them. Take extra care to be considered and civil. If you do receive downvotes, consider them as feedback that you could do more to better express yourself. There's nothing you can really do about those who downvoted for disagreement. Commenting on that will definitely result in more downvotes, from what I've observed.
https://news.ycombinator.com/item?id=117171
Based on your green colored name, you may not know that pg is Paul Graham http://www.paulgraham.com/
It would makes sense to consider downvotes similarly, however they also grey out the comment, so they aren't symmetrical actions. I've pointed out 'pg's comment myself in the past, but don't consider it a great practice now. As you note, it's useful for historical context.