Our administrators have indicated that on the residency trail, e-mail is becoming the standard and an acceptable method.
Either is fine, but many interviewees prefer to send e-mail because they have specific details and names still fresh in memory when they first see a computer, and can personalize it better. Whichever you choose, somebody will likely print it off and put it in your file, or photocopy it and put it in your file.
Despite what others may say, it is a crucial part of any application process. Not only because it helps you, but because it is the respectful thing to do after somebody has put time into researching and learning about you, and taken their day to speak with you on the school's behalf.