my understanding is that in recent years second looks had become increasingly "required" - meaning that though programs weren't overtly stating it, they were starting to rely on them to gauge applicant interest in the program. and it was a huge added stress and expense for the applicant. that's why the nrmp sent out a mailing to the programs this season (which our dean sent to us), stating that second looks are not to be requested of applicants and shouldn't place any students at an advantage/disadvantage.
having said that, i still think many programs encourage second looks. and if you aren't going to some of your top programs, i might contact them expressing interest and explaining why you aren't visiting (funds, time, etc.). personally, i am planning on going to re-visit 2 programs, not only to express interest but also to figure out which of them i am going to rank first...