i've heard 2 different views.
1) interview early so you and the programs aren't burnt out. don't schedule your top choices first though, so you can get into the feel of things. many of the interviewers (and you!) will be tired of asking and answering the same questions by the end of the interview trail. i heard this from faculty who review applications.
2) interview late so you will be "fresh" in the programs minds. this is from iserson's book, so i'm not sure how accurate it may be. they say that programs tend to evaluate earlier candidates on a tougher scale because they are comparing you to the "ideal" candidate. later in the process they may be more generous, after seeing the actual candidates. i think this is assuming you're above average though.
personally, i think it doesn't matter too much either way. just whatever fits your schedule. i would say, after doing 3 interviews, you do get tired, so don't schedule your top choices at the very end. it's hard to be enthusiastic for 2 straight months after 30-something tours of hospitals.
