If your school doesn't require you to participate in the match, you don't have to but it would be foolish IMHO for the following reasons:
1) you have no way of knowing whether or not there will be desirable unfilled spots left after the match
2) if you do not enroll in the match, you must obtain the list of Unfilled programs from someone who has (or from a web site that will list them). An access code is required to get the list from the NRMP web site.
3) you cannot guarantee that you will get one of those unfilled spots (just as there is no guarantee that you will match either)
and the BIGGIE:
4) just because a post is listed as Unfilled in the match does not mean it was unfilled or that the program chooses to fill it. Any positions offered outside of the match (ie, to IMGs, DOs, Canadian students, physician graduates) will be listed as Unfilled; programs may decide not to rank everyone they've interviewed or may decide not to fill the position - either because they didn't care for any of the candidates, they've given the funding to another department (and don't actually have the position) or some other reason. They are not required to offer all their spots through the NRMP.
It is very possible that you could rank a higher tier program but match at a mid-tier program, while the higher tier program goes unfilled. If that program did not list you on their ROL, you will not get the spot regardless of who high you rank them.
Hope this helps and I'll move it to the Rotations and Residencies Forum for ya!
