Including desired rank in a cover letter for an open-rank faculty position?
I suspect this is one of these questions where there is no truly "correct answer" as you can't predict what's going on in any given search committee, but I think you're unlikely to do any damage by leaving it open.  At least my understanding of these things is that a department would rather hire you w/o tenure (note that this is not quite the same as the associate/assistant distinction) as it is less of a headache with the administration; you'll get tenure if they really want you, and don't think they'll get you otherwise.  I don't think it will matter much what you wrote in your cover letter.  Concentrate on making them want you, and then worry about what they have to do to get you.