Yes, we should "agree" or "disagree" with opinions as opinions are less than certain statements. To such a question is a bit tautological, and thus it should be obvious that when an opinion is stated, one generally agrees or disagrees. To suggest otherwise would be to suggest opinions can be fact.
Although some people's opinions aren't just that. They have researched the subject thoroughly and have proof to support their side. Of course you should agree or disagree, what other choice is there? It is nice to converse with people who have opposing opinions to yours. I learn more from opposing opinions than by other means. I enjoy learning how other people think about things.