The fact that you are talking about rates is an indication that you may have bypassed your best initial move by teaming with a local RealtorÂ® to help guide through the process!
Here's a very informative link extending the list of reasons why you should involve a RealtorÂ® when buying from a Builder: http://docs.Steven-Anthony.com/BuilderRealtorAgent.pdf
Unfortunately, the Builder may be reluctant to now pay commission to your RealtorÂ® because he/she did not "register" you.
As far as interest rate go, I agree with Ellie on both the strategy and Opes Advisors as a Mortgage Broker/Banker.
Also, you might find the following link quite informative regarding the source of you financing:
"Retail Banks vs. Mortgage Broker/Bankers"