you can do a second look anywhere. some places will say things like "well, if you're really interested, you should consider coming for a second look". those lines are killers b/c it makes you feel like you have to. i'm not sure if you do.
i did several second looks -- one at a place i was highly interested in and interviewed at early (nov), one was at a place that i wanted to rank highly but had a so so feeling after the interview, and one was a place that i liked but than found out things on the second look that made me place it lower.
if you've rotated at a place, you DON'T need to do a second look. a second look is an opportunity to get an idea of how things normally run (outside of the sometimes artificial and staged interview day). it's a chance to remind the programs of your interest (especially if you interviewed early on). however, if you've rotated they know you and you should know about them.
the second look is laid back but i know of some people who actually "blow" the second look. one person showed up in jeans and used her cell phone multiple times during a 1-2 hour stay.
i don't know about other fields but in EM, a second look usually entails dropping by the er for a few hours and talking to the residents. it gives you a good feel. if possible, try to do this in feb (i know -- just when you're sick of doing all the BS that goes with interviewing) and before the programs rank you (it's hard to know exactly when any program finalizes their rank list).
i personally wouldn't travel far for a second look (especailly if it entails flying and lots of $$$ which most of us don't have) but others may have differing opinions