It MIGHT help your USMLE step 1 score. Doing a clinical rotation would help you grasp clinical concepts and perhaps train you how to think when presented with a set of signs and symptoms. HOWEVER, the gain is minimal. It's best to take Step 1 right after you finish your basic sciences so that they're still fresh in your mind. Don't forget that step 1 is still a basic sciences exam. It tests basic science concepts through a clinical approach/presentation. So to sum up what I said, doing a rotation will give you a "clinical sense" that may help your score, but in terms of knowledge, everything you need to know for step 1 you should have already studied in your first 2 years.