Hi,
I went to med school to be a cardiac surgeon. Everyone will tell you about the benefits of having an open mind, but let me tell you a couple of benefits of having a trajectory:
1. You'll figure out faster if it's really for you.
a. you'll spend more time getting early exposure to the field (scrubbing in, shadowing office hours, recreational reading, whatever)
b. you'll probably do research or other activities (e.g. free clinic) in the field you're interested in
c. you'll get mentors in the field you want to go into
d. Because of a, b, and c, people will let you do things they don't usually let other people do, or get you involved in things other med students aren't usually involved in
2. If it is what you want to do, you'll be better prepared to do it
a. because you've been doing research in the field, you'll probably have publications; or, if you're volunteering in the free clinic as a first year, you might be running it later on.
b. because you have mentors in the field, you'll be directed towards things those people find important (e.g., are publications important, should you do a year out, what rotations should you sign up for, etc.)
c. because you hung out a lot as an MS1 and MS2, you'll shine as an MS3 when you do your rotations
3. If it turns out to be not what you want to do (happens) no biggie -- a lot of the stuff you built up in 2 is transferable.
4. You might find options are open to you that aren't open to people who decide in the usual timeframe (e.g., cardiology fast track at Mayo, cardiac surgery integrated program at Penn or Stanford, etc.)
5. When you go through your clerkships, you'll get more out of them -- if you tell the psych people "I'm going to be a neurosurgeon" they're going to try to teach you what they want every neurosurgeon to know; if you tell the surgeons, "I'm going into emergency medicine" they'll teach you what they think an ED physician should know.
A couple of pieces of advice if you taking this path (e.g., "I'm going to do surgery until proven otherwise", rather than "I think surgery might be for me, but I'm keeping my options open") is to take the rotation you think you want to do FIRST. That way, you aren't all the way at the end of your clerkships in a bind if you find out you don't want to do it -- you have another nine months to figure out what you want to do. As far as the idea that you might perform better if you do it later, realize (1) you're going to be really gung ho at the beginning of the year, and that will make up for a lot; and, (2) you should have been hanging out a lot during years 1 and 2, so that you're going to look like a star on that clerkship even if you do it first. Second piece of advice -- DO NOT blow off ANYTHING because "I'm not going to need that. Approach it with "this is the last time I'm going to see this or get a chance to learn this, because I'm never doing peds/psych/surgery/whatever again". First, you aren't wise enough yet to decide what you do and don't need to know; second, you might change you're mind; third, it's more fun that way for both you and your resident.
Anyway, I realize it's not the common opinion on this matter, but wanted to offer another perspective.
Cheers,
AJ