You can NEVER go wrong addressing a letter to the CP or the DO. In the end, it's typically that person that determines who gets hired. Recruiters at any company (that you want to work for) understand that and won't penalize you for addressing it "above their paygrade."

Whatever you choose, just make sure that you have their Name and Title correct. That's more important than who you send the letter to (within reason of course... a letter to Joe Ramper probably won't help you get hired, but then again- you never know!).