Keeping tabs on the builders for this type of situation pays off.
Also, having an agent represent you will make the whole transaction go more smoothly, as you will have representation of your own. Having a buyer's agent present can prevent interest rate hikes at the last minute. Builders often require you to use their lender in order to get incentives and the builder's lenders are famous for interest rate hikes on their captive audiences.
How negotiable the builder is will depend upon several factors including: market conditions, availability and what stage of construction the home is in. You are more likely to receive a discount if the home is standing inventory.
Please keep in mind on a new home, the track agent represents the seller and does not represent the Buyer. I highly recommend working with a Real Estate Agent. Generally, there is no fee to the Buyer to have an agent represent you.