I didn't match at either of the programs that I did an externship, but I didn't end up putting either first and I matched at my No. 1. I have heard of at least ten to fifteen people that I know or keep in contact with from the interview trail that matched somewhere that they did rotate.
Remember, if you look and act better on paper than you are in person, then don't even think about going to your top few choices. You could really shoot yourself in the foot. I saw and heard of plenty of people that hurt themselves more by going to a program for a month than they possibly could have in an eight hour interview. )

Definitely do an externship SOMEWHERE! It is a hassel to arrange it, but completely worth it.

I did two. One at UCDMC to check out the program, one at Utah, because of family and since they only interview 20, to improve my chances of getting an interview there.

Both are great programs, but when I interviewed, there were programs that I liked more.

Would I have matched there? Don't know, I ranked them #5 and #2 , and matched at my No. 1.

If nothing else, seeing how medicine is practiced in a different part of the country is worth the effort of doing an externship.

Good luck,