My advice would be to first apply to all the programs that you may be interested in and get interviews. Then you can pick among those which ones you still are interested in based on whichever criteria you want. If you want to save money, you can just go to the ones most convenient by date or location.
I would caution anyone in any field against applying to the bare minimum number of programs up front. There are people who have done this, and i'm sure there are still some that will, but it is extremely risky. For example, if you only rank 5 PM&R programs, you statistically start with a ~15% of not matching if you're a US grad based on the 2009 match (
http://www.nrmp.org/data/chartingoutcomes2009v3.pdf), and ~40% if you're an IMG. It could be possible that only 3 will offer you interviews up front and if you ranked just hem, it would raise the risk to 20/60%. It is true that you could scramble, but there were 9 total PGY 2 positions and 7 PGY1, which could be of any quality in any place (and filled by any one in the scramble). Moreover, the scramble sucks. Is this risk comfortable to you?
The same statistics show diminishing returns right at about 8-10 ranks for US grads. If you interview and rank 10 or more PM&R programs, your chances of not matching drop to <5%, and ranking more decreases this risk only very slightly. These are probably the numbers of ranks that applicants need to keep in mind.
PM&R is more competitive than it has been in the past years without a doubt. Residency programs don't have to rank as many people to fill their lists, and its anyone's guess how much more competitive it will be this year.
You could argue that you feel that you could be an excellent candidate based on your grades, research, etc. and don't need to rank that many programs. That may be true, and those things will help. But, PM&R is a little different than most specialties, and they weigh heavily on immeasurable things like personality, interest in the field, past experiences, teamwork and so on. Like many things in the field, you can't quantify that statistically, so many people who may not seem competitive based on numbers may be very desirable to some programs because they have those qualities, and vice versa.
I would keep all this in mind, especially if you are applying to some of the programs whose names get tossed around on SDN a lot, because that's where it's competitive.
So, hedge your bets and apply to more than you think you need. You can always turn down an interview, but you can't always ask for one if you need it.
