Neil is right about when to take step 2. As an IMG, it is more difficult to get a residency than a US grad in general. A big problem is that there are many potential roadblocks for IMG's including visas and failing the US exams. Most PD's are worried about wheither you will even be able to join the program if they rank you. While the visa issue is a whole other discussion, you can ease a PD's mind a lot by having ALL of your exams completed and scores reported when you apply.
For IMG's (and in general) it is best to apply to programs on the first day possible, meaning Sept 1st. I do not mean to imply that if you do not apply on Sept 1st that you will not get a residency, but you will likely get the best response if you apply very early and have all your exams completed.
So, regardless of your rotation schedule, if you plan to match in the following march, you need to have taken your tests by at least the end of june or early july of the year prior to the match. That will give enough time for your results to come back before you send out your applications (barring any major delays).
Some may disagree with this advice, but this is how I did it and it seems to make things a bit easier when compared to a lot of the stories I have heard on the interview trail.
Good luck