From what I've seen in HN and IMHO, a well expressed opinion on both sides is welcome. If you do have something to add on the other side of a topic those are way better on a comment. Then a civilized discussion happens many times. That is one of the great things of HN!
However, any article/comment insulting anyone or any political opinion has its well deserved downvotes. I would expect that on both left-wing and right-wing opinions. I've downvoted comments I agree with and upvoted comments I disagree with, based solely on the respect shown and the quality of them.
Note: there are of course always exceptions, my description above is when comparing HN to the rest of the web