Everything is negotiable.
Bear in mind, a GOOD agent will make the money (and more) back for you. 1% difference in commission is about $8000. If you have a good negotiator working for you, you'll probably easily make more than the $8000 difference.
I offer commission breaks when I know I'm doing more than one 'side'. That is, if you will let me help you find your next place, I'll cut my selling commission to you by 1%. It's just good business.
But interestingly enough, I don't have to discount my commission very often. The reason is because I have a full blown marketing campaign that covers ALL the bases. If you're going to pay for a full commisssion, it should be because you're getting full service. If you're in a very hot market, then find a discounter who will put it in the MLS. DFW Texas isn't white hot like some parts of the country, and it actually takes a little work to get it sold.
So really it comes down to your market.
One other negotiating tip: Don't "offer" an agent. Have them "offer" you. Some of them will likely roll over on you and give you a heavily discounted rate. Beware the ones who discount too much, too fast. If they can't negotiate worth a darn with you, how are they going to do when a buyer's agent like me comes along! :)
Good luck.