At UHS you have to have a very good reason. For example I know of one of my classmates who deferred because his fiancee (now wife) was interviewing for admission the next year. They wanted to go through school together and they are now classmates. I also know of a woman who was supposed to start with us but was going to have a baby in our first month of class so she deferred and will be starting in the fall.
Ditto the others ... a good reason is key. I deferred UMDNJ-SOM to do an additional year of research prior to entering school. Also, if you've been away from school/academia/science for some time, even a year, they (UMDNJ-SOM) may require their summer pre-matriculation program. My situation does not require attending the summer program. Check with the individual schools you are interested in for their requirements.