just for clarification - i assume you mean as a 3rd year waiting for them to finish the senior classes rank to start contacting directors for rotations to do when you are a 4th year. and really it should matter too much - the earlier the better though if its a competitive field/away rotation to make sure you get it. also most clerkships are not run by the residency program director, look at their med school website to find out who the contact person is. if you just contact the residency program director they might not be nice enough/have the time to redirect you to the correct person.
i started contacting an away program probably around march to schedule an away rotation, but applied to VSAS (i think it was due sometime in april). i found that classmates who waited until june/july to start applying couldn't get spots at good med schools because they were full.