RB is right--to an extent i think..
You do not necessarily need to do a month of EM at a place in order to turn heads.. For example, I finished this month's rotation a week ahead of time and so am roadtripping to the next program in which i am interested in just to check them out shake hands, and see what is up there..
sometimes it only takes a few days to assess each other..if you don't click, it ain't a match anyway.
the benefit of visiting program is 2 fold--
they get to see you, you get so see them, the town/city, the interactions of residents and interns with faculty.
the interview day can be sterile-everybody smiles..it is always helpful to see the real deal and make an informed decision..
just my 2 cents.
as for doing the same rotation over and over--not a good idea--true..be well rounded..so do radiology--equally important for ER , and visit the ED a few times to meet the program directors..simple as that.
good luck