I took the (old) MCAT before biochem and did well. However, I do feel that it hurt me somewhat. I only scored a 12 on the biological sciences section and I think that's because the last section was a section about carbohydrates which I did poorly on. I took it rushed because I wanted to take it before it changed to the new MCAT, but would definitely recommend waiting until you complete all the prerequisites. There's really no disadvantage (unless you're completing prereqs your senior year and really don't want to take a gap year, but in that case, ask yourself why you really don't want to take a gap year.)