Do you really need this backlink?

There is a very simple rule to decide, whether you really need a backlink from the given page, whether the site is worthy enough, so you will not need in the future to disavow it - and Google uses the same rule for sure.

The rule is: do you expect ANY traffic this backlink will generate? The real link value isn't a page's Page Rank, but a generated keyword-targeted traffic! If you see any site, you can decide with your common sense and without any analytic tool, whether you will get barely one single visitor from it. If common sense isn't enough, make use of free analytic tools like MajesticSEO, Open Site Explorer or the like to estimate the traffic. Try to estimate traffic not for the the main page, but for the page wherefrom you plan to get a backlink. And if not - you don't need such backlink. This rule is applicable to pages (not sites) without PR or with PR 0-1. You can assume, that pages with a hiegher PR do generate any traffic.