Check the policy for each place your going.
That's not a policy you can actually check. Any school is going to tell you that every interview matters and blah blah blah. I know some of you are getting nervous about your first interviews. Relax. Know the answers to the big questions:
1) Why MD/PhD?
2) Why not just MD? Why not just PhD?
3) Describe your research experiences. This is a really broad question. I always gave it a several minute overview of what I'd done in the past.
MD/PhD interviews come down to two types of interviews:
1) Interviews with director/asst. director/other adcom member. You will recognize this interview. You will do most of the talking and you will get most of the usual questions (Why MD/PhD?).
2) Interviews with lab heads. This is a recruiting tool to try to convince you to come to their school. They're practically rotation interviews really. Think of these as the types of people and the types of research that the school is trying to show off.
Typically, the grad program bows to whatever students the MD/PhD program wants. MD/PhD students are almost always more qualified than PhD students in numbers and research experience, and as such unless the grad program has a real problem with you, they don't put up much resistance. If the guy on the other side of the table talks 90% or more of the time and/or gets up and starts showing you their lab, you know which kind of interview it is.
Every year, some of the more zelous interviewees actually try to read their interviewers research before coming. This is pretty much a waste of your time, because more than half the time you won't even get in a word edgewise on your interviewers. After a program or two, if you are one of these people you will almost certainly stop doing it. The real challenge is trying not to nod off in front of someone who drones on at you at 8AM or after your 14th interview.