Hi,
To clarify, our program is not MSTP. I am in Canada, but I am American and did my undergrad in the states. Mine is purely a grad student perspective. That being said, I would say that during my grad school recruitment visits and during the recruitment visits of prospective students that have occured since my arrival, interviews with PIs are about research. They will inundate you with information about their research and discuss potential projects in their labs. Then, they will take you on a brief tour and perhaps introduce you to some students. I have never seen anyone in a suit. Mostly, people dress nicely, i.e. dress pants and a nice shirt (ties are rare for guys).
In my opinion I don't think that PIs care what you wear, and in some cases will not notice either way (unless you were a complete slob or in a prom dress or something
). For student-hosted events, even if PIs are there, I would still say that it is casual. At my school, students have a say on every committee, including recruitment and admissions. Thus, if anyone appeared really unstable or a bad fit (and clothing would not define either of these), they could receive a thumbs down from the student rep on that committee, on behalf of the students. However, if someone wore a suit or something to a potluck at a students house, let's say, they might feel really uncomfortable. This could result in them being introverted and giving a bad impression. Does that make sense?
So, I guess what I am trying to say is, if your interview is related to admissions, then by all means wear a suit. But if you are interviewing prospective supervisors, it is most likely a bit more relaxed. Events targeted at meeting current students are designed to be casual and fun, so dress should be likewise.
I hope that helps! Also, I could be way off base compared to MSTP. Perhaps someone who is a few years in and has experienced both sides could provide some insight.
Good luck!
Treg