hi OP, this is what I think:
June - Early August Aim for High Power Programs Letter writers.
Early enough to get a high power letter (UCSF, MGH, RRC chairmen/members, ASA president, look them up to see where they are at!) ----->This letter will carry a lot of weight with ALL programs (if it is a good lettter, not the generic one paragrapher). However...they might not remember you as well when you come back for the interview.
Late August - Early October Aim for your Number One program.
Best time in my OP. You'll be able to get a Letter in time, do it early, take calls with letter writer. May be granted an interview ONSITE, thus saving you a trip later. But again, would you want your "first interview" at your top choices? I would schedule to come back to your number one in Late November-December when you are more seasoned and at your peak. Also show more commitment to the program in my OP. Think of it as a built in second look. [They will see you in late summer - fall, then again during the winter without wide gaps in between] This plan allows for the most continuous face time.
November - December even January Aim for the Region where you will be applying to with your number one.
I call this the wild card money saver time. Schedule this in a region where you plan on applying to a bunch of programs. (i.e. tri-state, NW, Chicago, Boston, Mid-Atlantic, Texas, Southern corridor/Route 66/Bible Belt etc) So when you are on your rotation, you can go to all of your interviews at the same time THUS SAVING YOU A CRAP LOAD OF MONEY. Trust me on this, interviewing will get very expensive. They will also remember you quite well during ranking time. Not alot of MS4 around to compete with you except newbie MS3s, thus making you an instant stud. The con is that it might be too late for a letter ....but for the visiting program, face time with the chair, PD, faculty, resident will be even better than a letter in my OP,(if you don't have a pathologic personality).
If your school allows for more elective times, you can try to do 2 aways (a bit excessive but if one is during interview season, might not be so bad if you have a bunch of interviews in the same area!)
Visiting rotation is a double-edge sword. But if you're humble, fun, friendly, nice, professional, & have great work ethics....should not be a problem! I had a great time during mine. Good luck bud.