I've been link building on the basis that any one-way backlink is a good link. Does anyone know of any evidence that low quality links negatively affect your ranking? E.g. links from directories which include spammy sites or sitewide links.
Its all about percentages. If all your links are crap then you won't rank highly. If 5% of them are crap then you will be fine.
OK, so you're ranked at say #20 and you then obtain lot of low quality links. Could that make your ranking go down to #30?
It wont affect your ranking at all, but it wont have any negative effect on your ranking. It depends on the pagerank of the page where your backlink is placed. Whether the page is indexed or not. Suppose you have two backlinks in page a and b. The page a has a pagerank 3 but the no of outgoing links are 25. But the pagerank of b is 2 but the outgoing links are 4 only. The page b will influence more than the page a.
Rubbish links can have a negative effect on your rankings, if the site is new and doesn't have many other links.