While it depends on the particular program you are hoping to attend, most programs do not require the GRE. Of all the schools to which I applied, none required the GRE for admission unless you were interested in BME. Some schools make the comment that you might consider taking the test if you'll be applying for external fellowships (HHMI, NSF, etc.). So, if you plan to take the GRE anyway, then go ahead and good luck on it. It probably won't hurt your chances with MD/PhD, and could quite possibly help if you score well, especially in the subject tests.