"tiers" matter greatly in fields such as Law, because there isn't a lot of oversight from credentialling authority (ABA), and it's fairly easy to open a school. Thus, employment opportunities to a great extent depend on your status as a grad from a "Tier 1, 2 or 3" school.
In medicine, it's another ballgame altogether. There's tight control, both with existing schools and with the overall # of schools. So you really can't say there's any "tiers" as such, because any grad from any accredited US school can in theory and practise apply for, and get, any residency.
Also, I've actually never seen any statistically valid evidence that some schools have students who consistently outperforms on the USMLE.
However, there's obviously some schools that are more famous than others. How big the group is is open for debate, but it usually includes the "household names" that your non-physician family have heard of before...