You want your backlinks to be dofollow. If a link has the tag=nofollow it is blocked by the search engine spiders. If it does not have either nofollow or dofollow it defaults to dofollow. So as long as you do not see nofollow i is worth posting. There are addons for your browser to make life easier and know what sites are dofollow.
When a link is no-follow, it is basically telling Search Engine spiders not to crawl that link, therefore it not being indexed by them. In terms of link building, you would always want to get your link out where it is do-follow.
Correct me if I'm wrong... I think I've read somewhere in this forum that having a balance dofollow and nofollow links can do good for a link building campaign.
Do follow is very important but no follow as well. No follow do have some benefits in the SERP but most of the webmasters i know prefer do follow links ONLY. Kind regards, Yogesh