Is there a requirement that YC announcements perform well? I'm not trying to be negative, but just announce all the tours the same way and let people make of it what they will.
I can't imagine the amount of crying from the community if the hosts of the site would take special privilege to pin their posts at the top. (I'd be fine with it, as it's their house, but I think they actually are better served to only take a small privilege with the jobs and show posts and otherwise leave the site to the users to vote/curate.)