I gave a talk on my book today at John Jay College. After the talk a man came up to me asking about my use of "cop." He said when he was a kid, it was considered a bad word. Police officer is the proper term.
A couple of cops and students present discussed the issue. I don't consider "cop" a bad word. Maybe it is a generational thing. And also, as one cop said, "it's OK for cops to use the word."
On the street, if somebody addressed me in the second person as "cop," I wouldn't have taken kindly to it. Officer was my title and it is, to some extent, a title of respect. But if you say, "I called 911 earlier and this cop came and said...", that wouldn't bother me at all.
I think cop is perfectly OK as a descriptive and when used in the third-person. But no, you shouldn't address a cop as "cop."