If I write about another article, where should I link to the original source?
It does not matter where the link is from the SEO point of view. The link still flows PageRank wherever the link is be it at the top or the bottom.
From the personal point of view, Matt’s preference is to see the link close to the top so that he knows where the original source is from the outset.
Matt also makes the point that it is appropriate to give credit where credit is due.