Some people say it's a bad idea, some people say it's not a bad thing, most people say that basically it depends. When I say "it depends," there are several factors that are usually taken into account. First of all, it depends what you do during that year. Most people say it's a bad idea to spend a year on the beach or "finding yourself" or because you're burned out and just need time to yourself. It can be helpful if you do research for a year if you're pursuing something competitive and/or a specialty in which research is key. Doing some kind of medical mission is often a very well-accepted reason for taking a year off if it's something you're passionate about; I know a person from my school who did exactly that, and she got her first choice in a fairly specialized field. From the med/peds perspective, I actually talked to our PD about the topic. She was very encouraging about it as long as your reasons are good ones and you're prepared to discuss (and hopefully not defend) your decision-making. Got similar reactions from peds and pathology. Also, I think in many fields people are more understanding that stuff happens - people get sick, family members get sick or die, applicants have babies, etc. However, it seems that some fields are not as accepting as others, though I am reluctant to comment on which ones because I have only heresay (and not personal experience) regarding said fields.
I myself am deferring residency for a year, hoping to go into pathology. I'm doing it for several reasons - changing my mind too late in the ERAS/interview season, getting pregnant with baby due in July (what a way to start residency), and a husband with one more year in professional school in the same city (didn't want to risk breaking up our family - we have a young son). I've talked about this topic with various advisors and PDs in different fields, and the vast majority have not viewed it negatively as long as you do something medically related or academic during your time off - shows that you're dedicated, hardworking, and helps you keep your brain working. In my time off I'm working on a reseach project with an attending at my institution, and planning to attend various educational conferences at said institution.
I hope that helps a little. Don't be afraid to talk to people at your school, especially PDs to get their input. Good luck!
