I don't think there needs to be any official guideline about that. It's a community thing, and the community will vote based on whether it's interesting and whether the poster is known. I don't think those veiled SEO announcements get too many upvotes (unless said SEO black hats also commandeer an army of bots around here).

Personally, I've done a bunch of "Show HN" posts. A few of them did great (one went to #1 with over a hundred upvotes), while others only got one or two votes. That's how it should be, I guess.