When Google's disavow tool first came out, Google's direction was that we should only disavow bad links that we were responsible for. At that time, Penguin seemed pretty binary - you were hit or you weren't.
We have been repeatedly assured that there is no such thing as Negative SEO.
Now Matt Cutts has commented that a particular site has a "mild case of Penguin", and he advised the owner to "continue cleaning backlinks".
I'm getting a little worried here because I get a tremendous amount of bogus links to my site. Most of them are from Chinese sites, and I can only see them because they very often link to bad pages, causing 404-errors in Google WMT.
Another category of bogus links are sites that copy Wikipedia. My site is heavily referenced in Wikipedia, and there are a ton of sites that just scrape Wikipedia and reproduce the content.
When I do a "download latest links" report from WMT, and pick just a single day, I got 2,190 backlinks. Of those, at least 1,700 are from Wiki scrapers (most of them are from Poland). That's 78% bogus links.
I am not responsible for any of these links - I don't do or use SEO, I don't exchange links with anyone, my site has been around for 15 years and I don't need any boost that this kind of link trade brings.
But I'm worried because I'm seeing such a high volume of bad links, day after day, and I'm seeing a lot of new Chinese links which are not Wiki-based, but are very spammy (they seem to link to Chinese #*$! sites).
Which direction should I go? I don't think I'm currently being hit by Penguin, but if you can have a "mild case" of it, then that may change things. On the other hand, I would hate to spend my energy on disavowing a ridiculous amount of bogus backlinks which accumulate on the scale of tens of thousands per week.