If you intend to apply to a top-ten (MGH, The Brigham, Columbia, Hopkins, Penn...) internal medicine program for internship, then you have to do your medicine sub-I relatively early.
I was choosing between radiation oncology and medical oncology, so I did a medicine sub-I and a medical oncology sub-I relatively early. I did my home rad onc rotation very early. I still had time from July through September for potential away rotations. There are threads about away rotations already, but Ill get on my soap box and scream loudly again -- if you have a strong home institution, then you should really think long and hard about away rotations. I promise to be a bit more explicit on the topic of away rotations after match day.
For people going into specialties (medicine, surgery, ER, Ob/Gyn) where away rotations are essentially sub-I's then I would agree that you need to do your home sub-I first to get your feet underneath of you. When you go to a new institution you can't even find the bathroom, let alone the core lab, micro lab, pathology, ER, radiology--and that is half of what being a sub-I is about. At least after doing your home sub-I you would be competent at patient management--the other half of being a sub-I.