Remember, this is YOUR money that you are spending, so if you are smart enough to negociate a better deal, good for you. (Don't worry about the dealer, they'll take care of themselves! If they don't like the offer, they'll refuse it.) I don't think etiquette is really an issue, as this is business.
Typically, you can expect a 10% discount or so, especially when buying an entire system. (However, depending on the dealer, you can sometimes negociate even more. As an example, my friend negociated a higher discount on a set of very high end mono block amps ($20K retail and he got them for $17K out the door.)
One thing to consider is that your standard B&M store has a high overhead, so they have a slimmer margin, versus say a dealer that operates out of their house (such as the one my friend dealt with!).
Now, if the dealer comes to your house to setup the system, and fine tune it, then the discount might nothing, or very little, as they are not only selling you a component, or a system, but providing their time and expertise as well.
My two cents worth.
