perpetual did a great job of summarizing the issue and I think I would agree with everything he said. I would recommend taking it late enough that you are not forced to put it on your application in case it goes bad, but early enough that the score has the potential to help you if it goes well. Plus, it is nice to cruise the rest of the year and not have to worry about that test. If anyone ever finds themselves in the predicament where they took the deuce too early and score poorly, there MAY be a way to release your Step I individually to programs outside of ERAS. It may be an urban legend and I dont know anyone who has done it, but I recall one of my faculty members last year saying there was some way to circumvent the system if you got in a bind. It sounds a little hard to believe and at least borderline dishonest, but I guess if someone was in a bad enough spot, it may be worth exploring.
My opinion is that your step II score can help depending on your step I. If you are less than 220 I think you basically have to take it. In the 220-230 range you should strongly consider it if you can put together some time to study for it. In the 230-240 range it is probably optional but you may be more likely to hurt yourself than help. In the >240 range, it is probably advisable to wait. I was in the 230s and bumped up my score enough that people commented on it and I was glad that I did it. I have a buddy who bumped his score up dramatically from the 220 range and know that it did nothing but help him. Heck, I even know a guy who went from a 260 step I to a 280 step II and I am sure that didnt hurt (although this obviously cant be typical). The flip side is that I also know a couple of people who dropped their score and subsequently had a pretty high sphincter tone until the match. As perpetual said, your test scores are not the only part of your application and you should obviously take into account the relative strength of the rest of your application and how much work you are realistically going to put forth studying for step II.
The ortho match is tough and all of the applicants are so stellar that every little bit counts. Doing well on Step II just adds more proof that you are a consummate overachiever and that you will be likely to do well on boards whether it is true or not.