Using an agent with a builder is a good idea - but only an agent who can be of help. Builders mostly deal with Realtors on a first contact basis. If you did not come in with one, they will likely not allow you to use one. Builder reps will likely remember you, as there are not that many people coming into their offices these days.
However, if you are buying, everything is negotiable. There are quite a few things you could use help with when dealing with a builder. It will save you money in the long run.
I worked for builders for 15 years and can help you with the ins and outs. Please feel free to give me a call.
RE Marketing Consultants, Inc.