Tell them. The only email worth sending in this process is the #1 ranking email. It may have no effect, may have a positive effect, but most certainly will not have a negative effect. Programs want the best candidates, but they also want to match the candidates most enthusiastic about coming to their program and avoid matching someone who maybe didn't want to attend but matched because they fell down far on their list. I'd talk to your closest derm mentor about this. Sometimes if they have a friend/colleague at that program they can help scope out the best way to do this and / or do this for you while again advocating for how awesome you are.
The sooner the better. Don't wait if u really have made up your mind.