Since Google has launched the disavow tool for removing unwanted links, there have been some debates on whether every site should establish and submit a disavow request, especially you received the unnatural link notice in Google Webmaster Tools.
Some webmaster argued that you should do it even you receive no warning from Google, simply because the negative SEO is effective and then by establishing a disavow file you could show Google you are in control of your links. To me, you should do it if you were engaging with web spam techniques and got penalized by Google, whether it’s a manual action or the action trigger the spam filters.
It’s a smart move from Google. Imagine the first Penguin update hits hundreds and thousands of sites, and as a result, the spammers were scared away from the black hat techniques and keep their hands clean now. Google launched the disavow tool to encourage webmaster confess in front of the Google spam team. From now, Google will get more information about the spam related issues without much effort.
Good job, Google.