Many times I have heard reports that the driver said that the temperature gauge reading went up, and the driver was concerned, but did not stop to investigate. then the temp reading went down, and the driver thought all was well. Some time later, the engine blew. Just as was said above, once the water turns from liquid to vapor, the temperature sensor no longer provides an accurate reading (too low) and a false sense of security. If one wanted belt and suspenders (foolish, just watch the gauge), you could add a temperature sensor as used on air cooled engines.
Scott in CA