the best answer imho is a combination of what you all were saying.
1. Your school will limit you on how many you can do. I could only do two. When you can take step 2 will take this into account. We had to take step 2 by january. I wasn't going to take step 2 during an ortho month, or while I was interviewing.
2. Your personal preference for number of aways will come next, financial, family etc.
3. There is no formula after that. Aways can help you, hurt you, and be enlightening.
Expansion on #3:
1. If you're a positive, energetic person, and get along with everyone, all of the time...You'll be okay on any away rotation.
2. The problem with any away rotation is that your job there is to leave an impression. So that means stand out, but don't be anxious and pushy. Don't stand out for bad reasons (you were late, etc.) During 4 weeks it's hard to do that as a medical student, that is, leave a good impression. The main thing that the program gets from the away rotation is: I can work with this guy, or i cannot. If no one knows who you are, and you don't stand out... that can hurt you.
3. You can get away with doing NO rotations.. It only helps when you're interviewing at the places you rotate at, for something to discuss if asked, and to get letters.
I was enlightened. I realized I didn't want to goto the places that I rotated in. Go figure.
The problem with that realization is, that once you realize you don't want to go somewhere.. it's hard not to show it.
Bottom line is, if you're going to be tired.. and out of steam.. forget doing as many as you can. Goto aways that you want to learn about the program that you have no idea about, but maybe are interested. Aways at big institutions are not the biggest bang for your buck, because you are going to be lumped in with folks that are all going there.