My chairman says you count anything you are present for. I do not know of any published guidelines on the acgme site or otherwise. In the coming years you will see hospitals asking you or your program how many of such and such case you have done. If you haven't done enough for their quota they won't give you priveleges to do that case in their hospital. I know this sounds ridiculous but I guarantee you it is coming. It is in fact silly because there are way more cases out there than any given resident can do more than one of during a residency, but does that mean you aren't qualified to do it? absolutely not. I know that is a bit of a rant, but if I were you I would count every case you are a part of, even if you are just looking over someone's shoulder.
Also note that you should count fracture reductions done in the ER. I know the attending is rarely if ever present, but you should log them with the name of your staff on call that night. These are important orthopaedic procedures that you should get credit for doing, and when else will you get to log them. How often do you take someone to the OR for closed reduction and casting/splinting? almost never. Put them all in the computer man.
Of note, you cannot count putting in traction pins, but you should code for removing them. Another ridiculous vagary of the system.