For a variety of reasons, I've been thinking a lot about design and usability guidelines. I'm often asked, "What's the standard?" or "What's the best way to do such-and-such?" Well, as anyone who's worked in design knows, the answer is: It depends. Now, that's nothing new, but this morning I've formulated a new set of "über-guidelines", if you will.
1) If you have guidelines and they have been proven with users, follow them.
2) If you have guidelines and they have not been proven with users, follow them for consistency (and offer to prove--and improve--them).
3) If you do not have guidelines, develop and let users prove them.
4) If you do not have guidelines, nor the time to develop and prove them, the designer gets to pick. Just be consistent.
Beyond that, it's a matter of opinion, and the designer--who (by definition) is in the role of, um, "designer"--wins.