Really, I think it depends on the program. I read that some people here on SDN were invited to second looks at a date after their interview (can't remember if these were all FM programs or not, I tend to lurk for no reason). Those are probably limited invitations not given to every applicant.
All of the programs I interviewed at, however, clearly invited everyone back for a second look if they wished and emphasized that the second look had no bearing on their ranking. My attitude was that once interview day was over and I was out the door, the committee had already decided where to rank me. I only did second looks because I could not decide between my top 3 programs. Some of my programs had mixer type second looks with the option of a shadowing day, others invited you back for a shadowing day. IMO, second looks are really for the applicant, not the program, so only do second looks if you are unsure about your rank list come late January.
If programs all have different policies, you are not going to get a straight answer unfortunately. If you want to know about specific programs, people here may be able to give you an idea of what to expect.