This post is an etiquette post that mentions resteeming but it's the only one I've seen that addresses the resteeming issue. It kind of talks more about how it is or isn't to your advantage to resteem rather than whether it is polite, which may be why other etiquette posts don't include it.

I did a bunch of research before and just after I started steemit. The 'guide to success' posts don't generally mention resteeming either.