I think the best advice I can give you is to not let anyone make you feel guilty about having a backup, whether they're on SDN or a student or faculty from your school. Derm programs don't want you to have a backup, but it's no skin off their nose if you don't match and you get left out in the cold. And none of them will lose any sleep over you. Lots of people didn't match this cycle, some of them probably for the 2nd or 3rd time. That's devastating for those people, but all I've heard from programs is how happy they are that they matched student x and student y.
This is a game, and you didn't make the rules. All you can do is play the game to the best possible end for yourself. You determine what "best" is for you, but I'm a person who hates unknowns in my life. I would not be able to stand taking a year off or doing a research fellowship not knowing if I was just wasting my time or if I would ever get a derm spot. I have close to $200k in loans that are coming due soon, and I can't afford to get jerked around by a program about whether or not they're going to have a spot for me next year. You have to decide what's important to you and do what needs to be done to make it happen.