There's a conflicting opinion from a cop on Reddit. Basically he agrees that you should shut your mouth, not let them search your car just for the heck of it, etc ..., but when it comes to the question "Do you know why I stopped you?", playing dumb won't do you any favors. There's a lot of other useful info he says that are relevant to the thread.

http://www.reddit.com/r/everymanshouldknow/comments/1rsizj/e...