Let me share a story. I lived in a new development about 7 years ago. The neighbor lived in Florida, came out, saw the models and signed the contract. Toward the end of building he came out and noticed that a number of things on his contract weren't built in his model. There was supposed to be a very large balcony off of the master bedroom. Considering that the yards in this particular track were miniscule, the large balcony really made a difference in your outdoor living space. He was supposed to have a cabinet in the downstairs bathroom sink. Instead there was a pedistal sink. Since this was a full bath, and there was no other storage, the cabinet made a big difference. My neighbor didn't have anyone representing him. The builder's reply was, "Look, if you don't like it, then cancel the deal." At that time, property was escalating in value so quickly that they knew they could re-sell the property for more than the contract they had with my neighbor. He was left to take the property the way they decided to "give it" to him. If he had an agent to fight for him, I believe he would have gotten what he paid for and what his contract stated he was getting.
Prasad, be represented. Builders care for themselves, not you! It doesn't cost you any more, and in fact can save you many thousands of dollars!
That being said, yes, you can exclude a new home community, a for sale by owner,etc. Just be aware that if you demotivate the agent then they may not represent you to the best of their ability or give you the quality service you deserve. If they have 5 other buyers who are all exclusively represented and who know the value of the service offered, you may end up LAST to get a look at the real good deal.
Remember, every Seller has to sell the property three times; to you, the buyer, to the appraiser and to the Realtor community and the agents therein!! Builders know this and our offering big time incentives to get us to the table. You could instead negociate a certain fee for the agent with any overage being credited to you in escrow as an added Seller's credit. Just make sure your lender is kept informed of these credits as it may exceed their guidelines and cause issues with your loan. Goodluck!!