Obiously I can't answer your question as the question is as to whether my viewpoint or Gordon's is the "correct" one. But I can give you my reasons for saying it is room temperature.
1. As it is obviously somewhat arbitrary as to which temperature is chosen such matters are often left to a standards body. In brewing in the US that body is the ASBC. The ASBC's method for measuring wort pH (for example) says to collect the stuff, cool it to 5 -8 °C and then measure it's pH using the method of MOA Beer 9. MOA Beer 9 says to attemperate to room temperature. Thus I conclude that ASBC expects pH's to be measured and reported at room temperature. This also tells me that if you publish an article in the JASBC (the ASBC Journal) your pH values should be at lab temperature unless you state otherwise (and their are reasons to do this if talking about enzyme performance, for example). This is the strongest "legal" argument for room temp.
2. Room (laboratory) temperature has a historical basis. pH meters did not fit into shirt pockets in the days when people started measuring pH in breweries. Thus the sample was removed to the lab during which time it cooled. If the sample was wort it was treated as in 1.
3. If one is going to measure pH at temperature other than at room temperature that temperature needs to be stated. As pH shifts with temperature we need know what temperature. Is "mash temperature" a protein rest at 122 or 128 °F or a saccharification rest at 146 or 156°F?
4. This is probably the most important to home brewers: The thin glass in the traditional electrode or the fine wires in an ISFET are stressed by sudden temperature changes. Making all measurements (buffers and samples) at nearly the same temperature prolongs electrode life.
5. ATC is a wonderful thing provided the isoelectric pH of your electrode is between 6.5 and 7.5 and it should be. But if it isn't (I own an otherwise fine electrode with pHi = 8.4) then ATC will introduce appreciable error if the meter was calibrated with buffers at room temperature and then used to measure at "mash" temperature.
6. Jean DeClerck wrote: "When pH is mentioned in connection with mashing, it always refers to the cooled wort."
Given the above I think it is incumbent on anyone who makes a measurement at other than room temperature that he publish that temperature. This is not always done and we will always be subject to some confusion on this score until everyone gets on the lab temp (or process temp) bandwagon and the ASBC changes its MOA.
This is not to say that we wouldn't prefer to know what the pH is at the actual operation temperature. Reporting it that way is just not as practical as a lab temperature specification. Somewhat like the weather bureau reporting the Denver barometer referred to sea level.
dmtaylor wrote: You want 5.3 at room temperature, but if measured at mash temperature it will be a bit higher. Can anyone verify?
Actually its the other way around. It is lower at "mash" temperature and the number usually tossed out is 0.3 but they don't say what ''mash" temperature is. In my experience the shift is about 0.0055 pH/°C. Thus if room temp is 21 °C (that's what the thermostats in overseas hotel rooms seem to be set for) then a 0.3 ° shift would take us to 75.5 °C (167.9 °F) which is more like mash out than mash temperature. If mash temperature is 150 (65.5 °C) the temperature difference from room temperature would be (assuming your results are like mine and it varies with grist, water, flood stage of the Nile...) the shift would be more like 0.24 and the sample you meadured at 5.3 (too low - should be 5.4 or so - but not terribly low) at room temperature would be at about 5.06.
