Personally, I tend to include the first five pages, although this depends partly on the guidelines of the agent.
The advantage of not including the first five pages with your query is problem solving. If you have a query that is flying naked and getting no response, then you know your problem is in the query. Queries are relatively easy to look at and determine their problems.
However, when you throw in the first five pages, you don't know for sure where the problem is. It could be in the query, it could be in the first five pages.
Obviously, if you don't get tons of rejections, this isn't a problem. But if you get a lot of rejections (you get to decided the number for a lot of rejections), then you need to start figuring out what's wrong. Multi-layered problem solving is difficult. The more things you throw into the mix, the harder it will be to determine where the problem is.
That being well known to me, I still send the first five pages, and frequently a synopsis (to really complicate the question of what's going wrong).
Best of luck,
Jim Clark-Dawe