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?
This will vary from agency to agency, but in general, the 5 pages will either be ignored (if the query is so bad that we don't get that far) or actually read, depending on who's reading it. I'm paid by the hour, and I'm an insanely fast reader, so I'll read the pages. A very, very busy agent with no assistant might not, but 5 pages in manuscript format is really not a lot of text.
On the other hand, if I say, "Yeah, go ahead and send five pages," everyone who reads this will start doing it, and then it'll become a nuisance, which is why AgentQuery says that. So I answered your question, but I'm not going to take a hard line and tell everyone to send 5 pages or not. In general, listen to what the agency says they want. If they don't specify, just send a query.