I used to run a stat.
But as I have come to better appreciate just how much hotter cyl 4 walls get compared to the other 3 cyls, I no longer think it is a good idea. Cyl 4 water jacket needs all the water flow it can get, not less. Use an IR temp gun under various operating conditions and see for yourself. Henry was nice enough to have the actual left hand (driver's side) cyl walls right out there for our perusal. You will get a reading at least 20° hotter than the others, maybe even 30° after a hard run.
So, rather than impede the water flow in the water jacket, I would rather impede the air flow in the radiator by using cardboard or a piece of 1/8" plastic flat stock in front of the radiator. The stone guard hides it and besides I don't care about looks, I want it to work properly. In the cooler months here I usually end up covering up the bottom half of the radiator. You can experiment with your temp gun, or water temp gauge if you have one