I've observed that there is an important phenomena in mashing pH that needs to be spread to you brewers. Through the thousands of mashing pH measurements that I've made over the years, there is an interesting tendency in early and late pH measurements.
When an early mash pH measurement is lower than 5.4, the pH tends to rise toward 5.4 during the mash duration. Similarly, when an early mash pH measurement is higher than 5.4, the pH tends to drop toward 5.4 during the mash. For that reason, when you have done a reasonable job of treating your brewing water to produce a desirable pH via either experience or a program like Bru'n Water, its probably best not to screw with chasing pH when you find that an early measurement is off.
To produce the most homogeneous distribution of minerals and acid in your mash, its also very important to add those components to the water before mashing in so that you can thoroughly mix them into the water. Anyone that adds minerals or acids to the mash after the grain is in there, is not doing themselves much good. It takes an incredible amount of mash mixing to get those components evenly distributed at that point. The only brewers that can get away with adding acid or minerals to the mash after the mash-in are those with wort recirculation. Recirculation does a better job of mixing than physical mixing can do.