On the latest video of Matt Cutts, he answered a question regarding dofollow commenting system and how it could harm the reputation of a site. He said that if a site contains spammy comments and linking to low quality sites, this makes the site’s reputation negatively affected. Since dofollow blogs (blogs with comment system with links that are dofollow – link power/juice transfers) usually lure a lot of spammers around the web.
However, he said that commenting to dofollow blogs does not have any negative effect to the commenter or the site that will link to the dofollow site. It is because, a site can’t control anyone who will link to it. This is really a good news for link builders and SEO practioners.
By the way, here’s the video of Matt Cutt’s containing the said discussion about the dofollow blogs:
Now, my advice is that you turn off your dofollow commenting system. WordPress usually use an “external nofollow” tag to every link in the comment section of the post so WP bloggers shouldn’t not worry. However, if you are not sure if your site uses nofollow or dofollow, just right click any link using firefox on your comments then check if you can see an “external nofollow.” If you can see it then there’s nothing to worry.