According to the MSAR, almost no schools require Biochem outright. That said, almost everyone involved in this entire process (from SDN to pre-health advisers at my school, to current medical students) says take it. Most say take the best Biochem course you can find. From my (limited) understanding, most med schools cover it in the first two years. So the more you know about it, the better prepared you will be. Not to mention that the longer, graduate level if you can swing it, course looks better on your application than a survey. Just my two cents. I'm a post-bac, and looked very carefully at what I could cram into two years. A full year of biochem was on everyone's list.
EDIT: Important enough that I took gen chem and o-chem at the same time in order to fit a full year of biochem in. That important. Don't sacrifice your grades though, that won't help no matter what classes you take.