Well, what is written in Bru'n Water will be updated based on the mashing trials I've been conducting. I've seen this response for years in my mashes where the pH rises through the mashing period. As Larry said above and I've now confirmed without any doubt to me, pH does rise during the mash. Thirty minutes is definitely the earliest you should consider the mash pH to be somewhat valid. Forty five minutes is even more accurate. The pH becomes relatively constant after that time.
Measuring at 15 minutes AND planning on implementing any sort of pH adjustment is not a good approach. The pH SHOULD be significantly lower than your targeted pH at the 15 minute mark. You shouldn't be adding a base to your wort if you find that the mash pH is several tenths lower than you intend. The ONLY reason that I would consider an adjustment is if I measured a pH that is way over my target. If you're using a competent brewing water calculator, your mash pH should be a bit low at the 15 min mark and it will climb during the mash.
You can measure at 15 min, but it only matters at 30 and 45 minutes. The pH of the early mashing stage is less important. Making sure that your wort pH into the kettle is in the 5.2 to 5.6 range is what is more important.