I don't think geographic region matters as much as people think. The match list for BU has very few MGH and Brigham & women's candidates.
First, as discussed in other threads, regional bias is really about "nexus", will th person come if we rank him, based on his connection to the area. You can have nexuses from lots of different things besides college. Your family is here, your SO is here, you did multiple away rotations here, you did research here, etc. Second, the Harvard hospitals are big names in many specialties, so they have fewer real competitors for matches. They don't have the same concern that they won't get their top ten off their rank list as a smaller hospital might, so they won't really care if you are coming from Boston or Kansas. But the reverse, Kansas, is going to worry more if someone from Boston applies to them seemingly on a whim.
In truth there is a home school bias for a number of reasons. First some places have a policy of automatically granting it's home med students an interview. So those people will always generally be someplace on the rank list. Might not be top, but that's some slight statistical preference. Additionally, you are far more likely to have LORs that are coming from people the PD has heard of or knows. So that helps too. And if you networked right, you will know all your interviewers already before the interview, which can be big benefit, although that also means your rotation is really a month long job interview, which can be stressful.
But bear in mind that most people applying to med school dont really know what field they want to go into, end will change their minds at least once. No place is the best, or even solid, in every field. Every program has a glaring weakness, and that could be the field you ultimately decide you'd enjoy most. So picking a Med school to try and game where you want to end up in residency usually won't pan out. Even at the best ranked med schools people are mostly ranking other hospitals programs highest. Relatively few people never move. So I think if you are looking at med school based on where you want to do residency you probably are doing it wrong. In fact, as you progress in training, you will find that to end up in a certain place you sometimes have to take a very circuitous and counterintuitive route.
