It all depends on where and when you're planning on rotating. Most places have very few restrictions on who can rotate, how many at a time, etc. Many have no restrictions at all. Also, if your school allows you to go away in may and june, few other students will be rotating then, so you'll have your pick of places (and you'll be one-on-one with the whole program, so you can really shine) -- in july and august just about everyone is doing their electives, so the crowds become more of an issue.
As for me, I had to rearrange my schedule only because of my home program's restrictions on the number of students each month. As for away rotations, I had no problems getting exactly what I wanted when I wanted it. I applied to 2 aways, got them both without a problem. If you start asking around now to see where you want to go, then start making calls to the programs around april/may, you should be all set. Most places won't let you apply much earlier. But, set up your home program first, because that'll determine how your schedule plays out everywhere else. After that, the earlier you put your application in, the better chance you'll have of getting what you want. Consider that some of the big programs will have you rotate on one service for the 4 weeks, so getting yourself on the "right" service can help a lot when it comes time for letters and interviews.