jimbeetle - 4:55 pm on Oct 19, 2012 (gmt 0)
In the video Matt very much implied that folks who did not receive an unnatural links notice would not need to use the tool. Also, the first paragraph of the webmaster central blog post:
Today we’re introducing a tool that enables you to disavow links to your site. If you’ve been notified of a manual spam action based on “unnatural links” pointing to your site, this tool can help you address the issue. If you haven’t gotten this notification, this tool generally isn’t something you need to worry about.
And in the video Matt did not mention Penguin. But, of course, in the blog post they state:
If your site was affected by the Penguin algorithm update and you believe it might be because you built spammy or low-quality links to your site, you may want to look at your site's backlinks and disavow links that are the result of link schemes that violate Google's guidelines.
So I guess it comes down to 1) you received an unnatural links notice, or 2) you know you built spammy links, were hit by Penguin and your ranking tanked. (Google being Google and muddying the waters just enough to cause confusion.)
The one necessary step I left out in the post above is that from what Matt said in the video and the sample disavow file in the blog post is that Google wants to see an effort to remove spammy links *before* a disavow file is submitted.