What is the ideal boil pH?

In my opinion the ideal target for Mash pH (as measured at room temperature) is 5.6 (because peer reviewed literature from brewing masters such as Briggs and several others of his era indicates that they targeted an ideal of 5.4 Mash pH as measured at mash temperature), and the ideal Wort pH to target via acidification adjustment post mashing and just pre-boil (also as measured at room temperature) is 5.2, (with the range here being 5.0 to 5.2). At 5.0 boil pH hop utilization is diminished and hop presence is somewhat muted vs. 5.2. Higher than 5.2 at this juncture and you risk the potential of the yeast not bringing the final (un-carbonated) beer pH into the ideal range of 3.9-4.0 to 4.3-4.4, above which juncture the beer is subject to downstream bacterial contamination. Also, higher than 5.2 pH at this juncture and your beer will darken unnecessarily due to Maillard reactions occurring during the boil. 5.0 to 5.2 pH during the boil suppresses these color raising reactions.

According to Brewkaiser, the ideal boil pH (room temp sample pre boil) should be around 5.2-5.4. Much lower than that, and you'll reduce hop utilitilization, but much higher and the hop utiliziation increases, but the bitterness is harsher. (The same process that causes tannin extraction at higher pH in the mash is at play in the boil also.)

A higher pH in the boil also increases the rate of Maillard reactions, causing the beer to darken faster. So for light beers you would want to aim for a lower pH. The lower pH also increases protein coagulation so this has a double benefit.

After the boil, the pH will have dropped by 0.1-0.2. So the fermentor pH will be around 5.0-5.3. The yeast drop the pH of the wort to 4.3-4.6 to maintain an internally higher pH, which assists with nutrient uptake, something that becomes progressively harder as the pH decreases, so avoiding a too low pH assists the yeast for a healthy ferment.

Finally, the ideal beer pH is 4.25-4.6 which gives the beer a crisper more lively taste. Higher pH results in the beer being perceived as dull. But much lower and the beer will start to taste tart and sour. (Naturally sour beers are much lower than 4.25pH.) A low or high boil pH will affect the final pH of the beer and thus the perceived flavor.