This subject keeps popping up and since I have been monitoring the temp pretty carefully (with additional thermocouple probe) over the years, I must say that the radiator cover has a place at ANY ambient temperature, depending on how the truck is used.
During my trip to Georgia and Florida a month ago, I kept the custom radiator cover in, even at ambients around 95°F. On the highway, the engine temp stayed at about 180-185 most of the time.
My custom radiator cover is equivalent to the canvas cover with the flap open.
I did remove the cover for the climb up the mountain coming back to keep the temp below 200° with a gross combined vehicle weight of 29,000 lb.
For short runs or stationary operations, I always use the cover (images elsewhere on this forum).
For your own satisfaction, install a second accurate, easy to read temperature probe/meter (I use a DVM w/thermocouple adapter) and find out for yourself.
Straight up and down on my dash gauge is 176°F, which means that for "normal" operation, the T-stat hasn't even begun to open.
I have posted this before and to you guys that are bothered by the repetition, I apologize.
PS. For highway driving with long downhill runs, I have seen the temp drop 40° in a few minutes without the radiator cover. Using it helps keep the temp much more constant. The ideal solution is a radiator shutter with a controlled fan....