I think if you are really interested in a program, second looks are a great idea. They allow you the opportunity to get a more in-depth look at a program and talk to more residents, and I think program directors take note of applicants who took the time to do second looks. This does not mean you should second look at every program you interviewed at, nor does it mean that if you don't do a second look you won't get ranked high by the program.
Some programs offer everyone second looks and some don't mention it at all. I think you should find out as much as you can about the programs you are really interested in and second-look at those which you feel like it would help, time-permitting. I second looked at my top two choices since I could not decide which to put #1 and #2. The residents at both programs told me that a second look would help a lot and that the admissions committee takes note of it.
If you interviewed early in the season, a second look may help to refresh the committee's memory as to who you are, and may also help if you did not have a great interview the first time around. Besides the extra time and expense, I don't see how a second look could hurt.
