Unless you have special circumstances, I don't think you really need to waste the money on applying for a prelim. Last year, I didn't match in ortho. On scramble day, I had three programs offer me a prelim position in g-surg programs. This was after only calling a few programs. Many prelim positions go unfilled. Also, you need to make sure you want to do a prelim year. It is not the only thing to do when you don't match. I am doing a research year since that was a deficient part of my application.
If you need to stay in one place if you don't match ortho, and are sure you want to do a prelim, you could apply to local ones just to increase your chances of doing so (you select the programs you are interested in, then click on prelim instead of categorical). However, how will you address why you are applying? I don't think saying you only want to go there if you can't get in anywhere else is going to get you a high spot on their rank list. Also, if you don't get different letters of rec and personal statement, it's going to look strange how everything references ortho.
Of course, in the end you need to do what you feel comfortable with. One final thought...For those that don't match, don't let your school officials force you into anything on scramble day. They want good match percentages so they will try to get you to go into something else (especially toward the end of the day). Stay true to yourself!
