Have you seen any signs of algorithmic penalties or SERP fluctuation? If not... forget it. Yes I would ask to have it removed if it's a garbage link from an irrelevant site. But I would avoid the disavow tool and use it as a last result. I don't trust that tool as much as others seem to... You can usually do well without it too. If you start getting manual penalty warnings and your SERPs are dropping without any response from the webmasters... then it might be time to break out the disavow.
Whatever you do, DOCUMENT it! All emails, contacts, discoveries.. keep these documented in a separate folder in case you ever have to submit a re-inclusion request to Google.
That's all I can really say.