Ok, just want to make sure I've got this straight since I'm an accountant and not a chemist. If I use a ph meter with temperature compensation, it is giving me a reading at "room temp" as long as my sample is within the temperature range the meter is rated for, right? For example. I have a Milwaukee ph56. The effective temp range is 23 to 140 F (-5 to 60 C). As long as I have my sample is cooled to at least 140F the reading I get will be as if the sample was at 68F.
A pH meter without ATC would read different at two different temperatures for two reasons.
1. The way the probe works is that temperature in and of itself affects the reading regardless of actual pH.
2. The actual pH is different at different temperatures.
The ATC attempts to correct for the first. I say attempts because the ATC assumes the probe is in perfect condition and the sub $100 probes most homebrewers use probably weren't all that perfect out of the box and certainly aren't after months of use. ATC does nothing to correct for the second. Even if your probe is perfect, if you read at 140 and then again at 75 you will get two different readings because of the second factor.
Furthermore if you measure at 140F you will place a great deal of stress on the probe and dramatically shorten its life. Very inexpensive probes are around $40 these days so my choice though I am not a very cheap homebrewer is to not going sticking them in hot stuff when I can just leave the sample in a small steel bowl in the freezer for a few minutes.
In terms of having consistent readings that you can compare from batch to batch, you should try to always measure at the same temperature. If you want readings that you can compare to literature or other brewers and you don't want to be buying probes every couple of months 25C is the best temperature to do the readings at.
My advice is measure at room temp and forget you ever heard about mash temp pH.