It's hard to study for Step 1 during rotations because you have to spend your valuable free time studying for the shelf exams (don't forget the importance of clerkship grades for residencies--these matter so much more than any grades you get during Years 1-2). Studying for shelf exams and Step 1 is not entirely mutually exclusive though--and the clinical experience will undoubtedly help you on many clinical parts of Step 1--but you really want some dedicated time for Step 1. Can you possibly set up 5-8 weeks of dedicated study time for Step 1 after the end of MS3? You can certainly make it work, but you'll have to use almost all of your available free time for Step 1, and that's really hard to do during year 3 when you get back after a long day and either want to relax a little or study for your shelf exams. Maybe even use christmas break to study. You'll have to find some way to keep up with some of the stuff not tested as much on Step 2 like biochem, physiology, etc. I'd talk to some advisors at your school or upper-classmen to get their perspectives too.