My thoughts, If you are going to install a heatsink then you might as well do it right. Heatsink compound will increase/improve surface contact area and improve heat transfer. How much? well even if its not much its still a little and would never hurt. Someone made an artical on it and said that is does help reduce heat. I forgot how much and where that artical is. I use a heatsink on my Xtreme 180 motors with compound and they don't even get hot after 10 minutes of flight.
You can find thermo transfer grease or heatsink compound at places like radio shack or most computer stores.