Unless you have a good plan, taking a year off is not a good idea. Some people may even think that you had a psychiatric problem or are weird. Be careful and develop a good plan before you take a year off.
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.
A well planned year can undoubtedly help your application.
I did research for a year at a top institution to be near a significant other.
With some good results and a sprinkling of luck, I was a first author in IOVS, a co-author in Science, plus had a couple of national presentations during this 1 year. It helped me invaluably in securing a spot at a consensus top program.