Obviously varied opinions will come from this question each and every time
I will agree that internal parts that make contact will 'wear in' over time and develop smoother mating surfaces with less friction that will enhance the smoothness of the action.
I will also state that replacing the trigger return spring and mainspring will do much more for you instantly! Then along with that the natural wearing of mating surfaces will 'self enhance'. Now, combine these both and you have a pretty amazing transformation in no time at all There are other areas of attention that a true gunsmith can address to better the action or simply send you gun in to Smith & Wesson for an action job. I'm pretty sure they will know just what to do to make you happy and keep the gun functioning properly for how you are going to shoot it.

Enjoy your Model 66, no matter what it will get better with time
Karl