It's generally proper etiquette to call and let them know. Also make sure you send them something in writing. They will be holding your seat until they are informed otherwise, so until you inform them,someone else will not be offered your seat. Make sure you are polite in your declination, as you never want to burn bridges.
Definitely let them know - professionalism counts. As soon as you let them know you won't be accepting their acceptance another student can be accepted. That really matters to that student.
Also, even if you call, you may want to send a letter thanking them for the interview, acceptance, etc. This is good because it shows in writing that the process is complete, and it also keeps a good relationship base for the future. Who knows if you'll want to do an elective at that school in the future, or even residency training.