Indicated pH is dependent upon the temperature of the liquid. In addition, there are other reasons why the user should standardize the temperature at which they perform their pH measurements.
pH meters will register differently as the temperature of the liquid varies. There are a couple of reasons why this occurs. The first is the difference in how the meter and it's electrochemistry operate with temperature variation. Manufacturers have incorporated temperature correction circuitry into higher-end meters to take care of that problem (temperature compensation). That feature is helpful, but it DOES NOT correct for the change in mash pH due to differing chemical activity related temperature change. The dissociation of ions in the mash varies with temperature and that creates an actual change in the mash pH. That change is reported to be up to 0.35 standard units, but both Kai Troester and AJ DeLange have reported that they have only observed about a 0.2 standard unit difference. In any case, the pH at mash temp will be lower than at room temperature.
So even with a temperature-compensated pH meter, you may not be reading the correct mash pH or getting a value that is useful.
The other reason that brewers should avoid measuring mash pH at mashing temperature is that the high thermal stress placed on the probe's thin glass bulb. Going from room temperature to mash temperature by plunging the probe into the mash will shorten the probe's life.
For the reasons above, pH measurement for the mash should be done at room temperature. There are fewer side effects in performing pH measurement at room temperature and it also means that you don't have to buy the more expensive temperature-compensated meter!