I got the feeling that 2nd looks were really more for the applicant. I'm not sure how much they move you up in the rankings. Though it always comes down to your comfort level in the end with what you want to do.
Another way of expressing interest in your number one is to tell them directly they are your first choice, to have someone (like a letter writer, Dean, etc.) also call or write the program director to confirm that a program is your first choice (so the PD knows you are really serious and asking someone "objective" to back you up), and e-mail or talk on the phone to residents to ask more questions (as that interest often gets communicated up to the PD). This is all most useful in January, probably, as the PD is putting together the rank list then and has an idea of where you will be on the list (though they may or may not give hints).
If you are able to travel, it is nice for your own piece of mind to see how the program looks on a non-interview day, and what the residents are like in their natural settings. But I still feel it is more for you than for the PD.
I did only one 2nd look, though I had done an elective at my first choice.
