Must I use the mortgage company that my builder directs me to?

No. It is your mortgage and you may decide upon the lender. However, most volume builders are effectively "forcing" their buyers to use their in-house mortgage company by refusing to pay certain fees or even altering upgrade packages based upon them getting or loosing the mortgage. This "forced-use" game most often spells higher interest rates for the buyer compared to what is available in the open marketplace.