Well, I'll add another 2 cents here then:
The only way to truly be sure that we are not overheating the engine is to plug-in a ScanGauge II and watch the engine and in-take temps. In other words, if the temps reach 90C (194F) and stay there or even move higher than that, then it is time to remove one row of blocking or more.
Now, I realize that not everyone is going to be buying a scangauge just so that they can watch the temps. Of course, a Scangauge (SG) can do far more than just watching temps, but it still costs $160+ shipping and some may not be able to make a case for it.
Anyhow, for those that do not have a scangauge and hence cannot tell the temps accurately (forget the dash temp gauge), there are other ways to tell when to start stripping the grille blocking. First, listen to your radiator fan.
I know, when you're in motion you can't really tell if it is running, but if you are not in motion or you put the car in "N" after turning off the gas engine and having the ignition in II, you should be able to hear it better.
In other words, if the fan runs, then the grille blocks comes off. Simple enough, eh?
In the name of prudence I only use grille blocking when the temperatures fall below 10-15C (50-56F) and I do so gradually by inserting the blocks in the lower row first. As the ambient temperatures sink, I insert more blocking. By the time the temps are below freezing I already have the entire intake blocked.
That's it in a nutshell.
Now the other interesting detail: I use a block heater all year round !!!
The only difference between a summer time plug-in and winter time use is the amount of time the block heater is actually on. That is why I use a timer. In the hight of summer I need less than two hours of plug-in time. In the cold winter days I need 6-8 hours.
Now, why would I use a block heater in the summer? Because the block heater raises the temps of the engine roughly 30C-40C above ambient. So lets say the ambient temps are 25C (77F) then the engine will have a temperature of ~60C (~140F).
Guess what that last temperature is?
Yep, many times I'll be EV gliding as soon as I am out my garage until I reach the main access road.
So using a block heater helps a lot in the winter but also in the summer as it helps bring the engine temps closest to its optimal operation range sooner.