Matt Cutts posted a video answering the question "Should I use the disavow tool even if there's not a manual action on my site?"
The short answer is yes. But when?
(1) When you get a manual action, of course.
(2) Webmasters won't remove the bad links to your site or want to charge you to remove them.
(3) You are worried about negative SEO.
(4) You see links pointing to your site you do not want to be associated with.
(5) You saw a link bomb attack and are afraid it might hurt your site.
(6) You are afraid someone will submit a spam report about you.
(7) You see your rankings dropped and you think it has to do with an algorithm Google ran, i.e. Penguin algorithm.
(8) You can’t sleep at night because of some of the links you have.
Here is the video:
Forum discussion at Google+.