I've had a Hanna meter for several years and it works great. I recently bought some new standards as my originals were getting a little old. Interestingly, the old ones still agreed with the new.
I quit using my meter for a while once I got Bru'nWater dialed in. But had to to back to it with Low-oxygen methods. I use yeast de-oxygenation for my strike water and the process definitely throws off some acid.
I did the same with my calibration standards that were getting old. When I checked, they were still reading the same as the new standards. I don't think that the 4 and 7 standards age much, but I do know that the 10 standard does have to be replaced frequently. Maybe that is why there is a general recommendation to change out your standards so frequently?
I hadn't considered that yeast de-oxygenation depresses pH, but of course yeast metabolism does produce acids and I guess I should expect it. The thing is that I don't expect the yeast action to have enough time to produce a significant pH reduction in the subsequent mash since you're only supposed to have the yeast and sugar in the water about 2 or 3 hours before use.
Assuming that your water quality inputs are accurate and the grains are typical, programs like Bru'n Water can do a pretty good job in assessing pH and helping brewers with the proper mineral or acid additions necessary for their brew. Its when your water quality strays or you're using atypical grains that it is helpful to double-check what a program predicts with an actual and reliable measurement. Since those programs predict room-temperature pH's, you do need to measure your wort at room temp. Those reported pH offsets vary widely and the temperature compensation function does not provide for the ultimate correction we want.
MEASURE AT ROOM-TEMPERATURE ONLY!!!