I would suspect that according to most state laws, a rider is not supposed to take thier hands off the handlebars when operating a motorcycle. If you wave, you do just that and some cops may pull you over.
A respectful nod of recognition is all I do. They do notice. I also think such more subtle and less emotional acknowledgement is more mature.
I do appreciate when a cop pulls up when I am taking a break by the side of the road, and asks if I need help, and I've expressed my appreciation right there and then.
I don't like when they hide in the bushes and point a radar gun at me, or follow me from a distance and wait for me to nail it.
So, I have a mixed feelings about them, and prefer a more reserved means of expressing my recognition.
Waving to a cop I don't know just doesn't feel right. Not to mention that waving to a cop who points a radar gun at you would seem like an ass-kissing.