My rules of thumb are:
1. Do what makes sense.
2. Do exactly what the publisher asks.
3. Do what makes sense.
Problems arise when what the publisher is asking for doesn't make sense.
If the publisher wants me to query for a two page picture book, I write a short query letter (which is mostly a cover letter, with a paragraph about the book). I say: "This is a query letter as you requested. But because my manuscript is very short, I've attached a copy with this letter should you care to read it." And then I either include a SASE or not according to their guidelines. Obviously, if they're at all intrigued they'll at least glance at the manuscript. So far, no one has objected to this.
Ellen Jackson
www.ellenjackson.net