I have some Ford tech material that says the CBV (Cooler Bypass Valve) is fully open when oil temp reaches 185 degrees. That's the oil out of the converter, the hottest oil in the trans.

The built-in temp sender is mounted in the valve body and is used by the PCM. The Edge Evolution, and doubtless other programmers, offer the ability to monitor TFT (Transmission Fluid Temperature) using this sensor. I live in NW Ohio and on a 30 degree day, TFT reads around 145. The converter won't lock up until TFT reaches about 85 (pretty much like clockwork for me) but I understand the PCW also has a timer built in. Form a cold start at around 30 to that 85 point takes about two miles for me... pretty much like clockwork

I've just started monitoring pan fluid temp on my truck as well in the bung provided by the Mag-Hytec deep pan and it readers lower than the built-in sensor by about 30 degrees. I will soon have a sensor in the cooler lines as well to see what the fluid coming out of the converter runs. (My name is Jim and I'm a gauge-o-haulic)

Bottom line to the original poster, not sure if I'd worry about covering the cooler unless you are a short hopper.