Dear Miss Snark:
Agent Query says, "Do NOT include sample chapters of your novel with your query UNLESS an agent's submission guidelines specifically SAY to include sample pages with your snail mail query. If you really feel compelled to show an agent your writing style along with your query letter, include only the first 5 pages of your novel." I'm not trying to just provoke a battle of experts here, but what do you (or most agents) think of that? If you were to recieve five pages with a query, would you read them? Ignore them? Shred the whole package and send it back in the SASE?
I always read pages unless the cover letter demonstrates a clear and compelling case not to. Most people can't write a decent query letter to save their lives. In fact, it was this assertion that led to the Birth of the Crapometer.
AQ is correct. Do NOT send chapters. We'll ask if we want them. Five pages max. Six if it means not cutting off a sentence.