if you take a year off, don't do it to study for USMLE!!! do it to travel, organize a preceptorship in another country, e.g. a clinical clerkship in medicinea at a hospital in mexico...if you can fit in some medical stuff into a year of travelling, you will have much to talk about at your residency interviews.
if you have good usmle scores, high class rank, you do not hurt your chances of a good residency by taking a year off.