I've heard mixed views. At my school's interview prep meeting yesterday they told us that if you don't show up, it doesn't look like you REALLY want to go the program (which may or may not be true). At the same time, many program websites say it is entirely optional and will not affect your application (though I imagine it could harm if you got drunk or came off as weird). Similarly, my former classmates who went through it last year and are now residents said they aren't necessary.
Regardless, I'm going to try to make all of mine, if at all possible, and certainly will do anything necessary to do so at my top 5 programs.