Hoople - 4:38 am on Jan 12, 2013 (gmt 0)

If the link makes sense for the user, do it! That is if the site linked to is considered an authority on the subject. Interlinking solely for the search engine's benefit is to be avoided if the target page is not an authoritative site.

Do link to the deep page (rather than the main page) where the answer exists. This should be a page that many would feel is authoritative on the sub topic.