Just my honest take, but I think majoring in Biology, Chemistry, or Physics can help you on the MCAT. You will take more courses in your major that go beyond the scope of the medical school pre-reqs and the MCAT, and that will inevitably help you once that test rolls around. For instance, I'm a Biology major, and even though Genetics is not a pre-req at most medical schools, my deeper understanding of the subject certainly helped me. I didn't have to spend much time studying for that subject on the MCAT, and I could focus on others more.
So yeah, honestly it does help you. But only a little. And, the little extra test prep isn't worth changing your major for. Pick a major that you enjoy and will be happy with. You may decide that you don't even want to do medicine. Who knows.