Of course, try to do the best you can in all you do (rhetorical statement award level sentence there, I know)... The grades, STEPs, pubs, and your personal and rec letters will help get you in the door. Connections made at an away rotation can substantially help as well here.
With that said, at the interview be able to explain "Why Derm?" in brief. Know how to sell yourself (I highly recommend the relevant portions of the book "60 Seconds and You're Hired"). What makes you relatively unique to the field, within reason? This is something I would not ignore. But there is a balancing act to play here, as you have yet to fully experience Dermatology residency first hand (i.e. you may have an idea of what you might specialize in within our specialty, but be open). I still maintain some of my pre-residency interests. However, I now have others I would have never thought of now that I am on the other side of the fence, being finished and all.
Transformers, I am happy to see you have hung onto your interest in Dermatology. Good luck buddy!