I would say that it varies greatly between programs, and even within the same program depending on the applicant. I have seen some programs where everyone who matched, did a second look. There are some programs who don't even formally offer second looks. Some programs encourage them, but don't require them. What I have seen most of is that there are certain applicants who they will take regardless, but other applicants who may be on the cuff, will need to do a second look if they want any chance to match there. Doing a second look, or showing interest in doing a second look certainly seems to be a wise choice, especially if the program goes out of their way to personally offer you one (it helps them to gauge who is truly interested in that particular program, as opposed someone who just wants a spot somewhere). With that in mind, my program matched all 8 spots this year... 6 of them came for a second look, and the 2 others attempted to arrange a second look, but where unable to make all the details work, and sent very personalized messages expressing their apologies for not being able to come. Having said that, I did not do a second look. The one program I did a second look at, I did not match, so take that for what it's worth.