If you use anything, use carbon assembly paste. Grease will reduce the friction of the seatpost on the frame, causing you to tighten it more than you would if you didn't use grease. This increased pressure on the carbon may cause failure of the carbon (cracks, etc..). Carbon assembly paste does the opposite of grease, in that it increases friction between the seatpost and the frame, causing you to need less pressure to tighten it.