I had a backup in another competitive specialty because it was something I was interested in.
When I was looking (and I suspect this is still the case), the derm re-match rate (for those who didn't match into derm their first time) was something abysmal like 8%. That's not a good cost/benefit ratio in my opinion for delaying residency (and repayment of debt, etc) to do research fellowships, etc. I also rotated at a midwestern derm program when I was a student who had a research fellow in her 4th year of "1 year" unpaid research fellowship. Seeing how they were treating her, and that she was willing to go along with it each time in the hopes of getting a derm spot probably soured me a bit.
If you decide to back up, I would lie about it. Interviewers in derm love to ask what your backup plan is, but the honest to God truth is that it's none of their business. If you're defaulting to your backup plan it means they didn't take you and their opinion doesn't matter. If they take you the whole point becomes moot. It's just a mind**** they like to pull. Make sure other students interested in derm or your backup field at your home school don't find out. It was tough for me, but I avoided most of my class for the 3 months leading up to the match because that's all anyone wanted to talk about and I was courting 2 departments. It seems like a dishonest way to do business perhaps, but my view was that if I went unmatched none of these programs were going to shed a tear for me or give me a second thought. You nor I created this cluster of a system, but you are still responsible for making sure you get out the other side intact.
Hope that helps.