Marie Haynes spotted two video responses from Google where Google’s John Mueller, said in general, it is best you nofollow links in stories you write, especially when those stories are guest blog posts for the purpose of link building.
In general, that is Google’s advice. If you link to something with the intent that it should help your Google rankings – then nofollow the link. If you write something without that intent and the link is really natural, then there is no reason to nofollow the link.
Here are the video snippets from Google’s John Mueller, start around 49:56 in:
Generally speaking, if you’re submitting articles for your website, or your clients’ websites and you’re including links to those websites there, then that’s probably something I’d nofollow because those aren’t essentially natural links from that website.
If you are journalist, you might be writing about a web site in a natural way… that is of course, okay.
With the second video, start around 31:19 in:
Think about whether or not this is a link that would be on that site if it weren’t for your actions there. Especially when it comes to guest blogging, that’s something where you are essentially placing links on other people’s sites together with this content, so that’s something I kind of shy away from purely from a linkbuilding point of view. I think sometimes it can make sense to guest blog on other peoples’ sites and drive some traffic to your site because people really liked what you are writing and they are interested in the topic and they click through that link to come to your website but those are probably the cases where you’d want to use something like a rel=nofollow on those links.
Google’s head of search spam, Matt Cutts, did post an official video on this exact question in October 2012.
In short, if you are guest blogging just to get links, then to be safe, nofollow the link.