To some extent, it probably depends on what kind of site you have. If you have a travel site about Elbonia, for example, you might have an Elbonian Hotels affiliate link on every page, along with an AdSense ad unit and a display ad. But you ight also have dedicated pages for specific affiliate products or services, with navigation "partner links" pointing to those pages.
I used to link directly to affiliate partners from my "partner links," and in a few cases, I still do. But in other cases, I link to a page about that affiliate partner and its offering, because that gives me more flexibility. (For example, if Widgetco Rent-a-Car decides to offer a doggie companion with every car rental, I can add a "Free cur with every car!" headline to [mysite.com...]