The Rule of Links is that you link when it's appropriate to do so. Linking is an art. It's a choice. You don't link from every word or even every noun, or from the subject of every sentence. But when a reader reasonably would want to know more about the subject, the Rule of Links says you should link to it.
This is something that is an art. I try to link to items that relate to the article, but it is very tempting to create links to everything.
As a bad example, I get e-Newsletters from Business2.0 that are over-loaded with links. I suspect they have a parser that creates a hyperlink for any term for which they have recent articles. This includes terms completely unrelated to the main body of the article.