In my mind, you shouldn't do colder plugs unless you need to. I think too often, it is on the list of mods when people are first modding as a need to do. It is not. If you are able to mod your car and not have knock, then there is no reason to run a colder plug. Your car will actually run better with a hotter plug.

Stick with the 606s until you need a colder plug.