Basically, if your grades are not good in undergraduate, if you want to do a masters to prove to schools that you can excell academically, certainly MPH or MS will help you.
But if your grades are great, there is no reason, you would be better off taking a year off doing something like you, teaching, researching, abroad or whatever.
If you are interested in MPH for real, then you should go for it. But it would make more sense for you to do it while you are doing MD or after because you will understand what kind of MPH you want do after you get more clinical and medical exposure in medical school, ie. biostatistics, empidermiology of infectious diseaes, or public health policies etc