It's funny. When I was at UCLA, I used to TA the summer school biochem class. Guess who was always in the class. MS 0 for USC.
Anyways, they are not strict about it, but if you want to do well on the boards, you should take it. My roommate is one of those who never took biochem, and he had to kill so much time studying for the boards on it.
I would say, buy Lippencotts Biochem and read through it right now. two years from now you will thank yourself for doing it.