I teach MCAT at TPR, and I tell my students that it is helpful, though not completely necessary. If you are taking a prep course (TPR or Kaplan) they will teach you enough Biochem and genetics for the MCAT. Biochem passages tend to be the most difficult (in my humble opinion) and the more biochem you know, the easier they become.
Also, I know of one med school (Michigan) that requires Biochem for admission, if you are considering not taking Biochem, make sure you are not knocking yourself out of the running for schools where you would want to go.