Short answer: No, but that doesn't mean its not a good idea to have one!
Most builders will require that your agent visit the sales office with you during your first visit to their community. Typically this is done to ensure there is an existing agency between the buyers and their agent. Upon arrival, the sales person will take your agent's card and register you (the buyer) as their client.
If all goes well and you are interested in purchasing a unit in the community, your real estate agent will be able to help advise on a good location in the community, share their insights on the builder, and help prepare you as you go through the builder's pre approval and reservation process. There may be incentives available towards the design center or towards closing costs. A great Realtor will do their best to negotiate with the builder to help their buyers get the best possible deal.
During construction of your new home, you will likely have a few dates where you will walk through your new home with the construction team to make sure everything looks the way it is supposed to. With your agent there, they may be able to spot things that you might not normally be on the look out for, and they can bring up the questions that need to be asked. I find often that part of the anxiety people have during the home buying process is over not knowing what to ask and making ill informed decisions. For me, I like to get as much info as I can from the builder/ seller and present it to my clients so that they have all of the facts before having to make a decision.
Finally, when it comes time to close escrow on your new home and agent can be helpful in coordinating the sign off and recommend professionals for any work you may want to have done at the property. Your agent can help recommend professionals for any upgrades you may want to have done at your new home.
At the end of the day, it is never a bad idea to have an advocate who can help make suggestions, answer any questions you might have along the way, and really look out for your best interests throughout the process.