I mostly work on content-heavy web sites (as opposed to more app-like products). I tend to assert some ground rules regarding primary navigation for menus for proper information design without having empirical evidence to back them. Some seem like common sense, but I cannot tell you how often I struggle with clients and developers over these.
So my question is which of the following guidelines have solid evidence to support them as best practices (if you can, please site the evidence, or at least industry-recognized expert opinion)
Only pages of this site should be included as links in primary nav menus. No PDFs, no external links (such links belong in alternate nav widgets, or inline)
In the best case, all main tabs should be clickable, providing at least a landing page (not just a for hovering to get a menu).
Consistent link behavior: for example, if main tabs are not clickable, they should all not be clickable. If if a sub-heading in a nav is clickable, all subheadings in navs should be clickable, etc.