C Lo eLoRiGiNaL is exactly right, the best way to get kills with the throwing knife is on Blood Money. I tend to do better getting the kills defending the base vault though, as opposed to the Money Pile, but it really depends on the map.
Good maps to do this on are ones where the base vault or money is inside a building and you can get high (like the Docks or Hollywood Heights). I've gotten most of my kills so far by hiding up high and firing down when I hear the "vault is being raided" prompt or see the money pile light up.
Finally, the throw action is very long, so to ensure a quick kill I like the hold the grenade button to "prime" the knife throw so you can kill someone in a quarter of the time it takes to make a full throw.