The problem with negotiating on your own is that it's always easier for a third party to get what is wanted. The recruiter is much more aware than you of what the market is like, and will get you the best salary. Or maybe you will accept a lower salary for different benefits like stock options, flexible hours or telecommuting. These things are best handled after you've received an offer.
Let the recruiter talk about those things instead of you. If you're going to work for a company, you want to come in with a positive attitude. You don't want to come in after going through a lot of tough negotiation thinking someone you're working for held you back from making a thousand dollars more.
Source: Commercial Programming Systems, February 10, 2000.