Tough question. I’d say, include sponsors where appropriate – either sitewide (on sidebars or footers), for some taxonomy or category (like on the sidebars of the blog), on a page by page level (have a contextual widget that will display a certain list of sponsors beneath posts related to those products), or just have one page listing all your sponsors, leaving the banner ads off the rest of the site.
No, is it appropriate? Would a hyperlink to an amazon product be considered a sponsorship or adding value to your readership? I’m not sure.