Yes, according to official statements from Google, if you don’t nofollow links in your guest post, you’re going against their guidelines:
Today we took action on a large guest blog network. A reminder about the spam risks of guest blogging: http://t.co/rc9O82fjfn
— Matt Cutts (@mattcutts) March 19, 2014
Guest posts used to be a very effective link building tactic. However, since it was abused by creating low-quality content, Google decided to penalize those who implemented “large-scale article campaigns”:
“Lately we’ve seen an increase in spammy links contained in articles referred to as contributor posts, guest posts, partner posts, or syndicated posts […] When Google detects that a website is publishing articles that contain spammy links, this may change Google’s perception of the quality of the site and could affect its ranking” – source (Google)
“The following are examples of link schemes which can negatively impact a site’s ranking in search results: […]: Large-scale article marketing or guest posting campaigns with keyword-rich anchor text links” source (Google)
“So stick a fork in it: guest blogging is done; it’s just gotten too spammy. In general, I wouldn’t recommend accepting a guest blog post unless you are willing to vouch for someone personally or know them well. Likewise, I wouldn’t recommend relying on guest posting, guest blogging sites, or guest blogging SEO as a link building strategy.” source (Matt Cutts)