Due to the process details which Stan Hieronymus revealed for the award winning line of high priced Ales brewed by the Trappist Monks of the Rochefort Monastery, within his book titled "Brew Like A Monk", wherein he detailed that they intentionally mash at a pH that always falls between 5.8 and 5.9, but subsequently they "mineral acid" (as opposed to organic acid) acidify during the boil, presumably targeting 5.2 pH with the objective of maximizing hot and cold break (which are maximized at pH 5.2), I believe there to be two primary objectives and thereby two potential addition points at which to acidify.
Prefacing this, from carefully reading the peer reviewed papers and documents of brewing masters of yore it appears to me that the presumption of targeting a nominal ideal 5.4 pH at room temperature is a misnomer, and the target of 5.4 pH was measured at mash temperature. Thus (in my opinion) the real "ideal" mash pH target in room temperature terms is more likely to be on the order of 5.6. In addition it is well known that mashing and even more so, boiling, at higher pH's ultimately leads to darker final beer color. We also know from no less an authority than Bamforth that mash yield (efficiency) is little impacted by mash pH, as the primary enzymes are rather pH resilient.
Back to Rochefort. Rochefort is not overall concerned with regard to achieving a finished beer at a low Pilsner like final color of about 3.5-4 SRM as is the demand for many Pilsner or Pils style lagers. Therefore they do not need to concern themselves with early acidification which is primarily intended to keep final beer color at its lightest. They thus skip this step. But boiling at higher pH's is attested to result in a more harsh hop bitterness, albeit that it does also result in measurably greater hop utilization, meaning that hop usage whereby to attain any given IBU target is lessened via keeping boil pH at the high end for as long as is possible.
Those who are most concerned as to maintaining low final beer color must acidify their mash water early (pre-mash). But Rochefort are only concerned to both maximize hot and cold break and enter fermentation at ~5.2 pH. If smooth (as opposed to harsh) bitterness is the goal then it seems that this latter boil acidification step should be done early, to perhaps (for the very smoothest of hop bitterness, with an acceptance of lower utilization requiring more hops to compensate) just pre boil.
The above said, I have to believe that when Rochefort claims to mash at between 5.8 and 5.9 pH they are speaking in room temperature pH measurement terms. Mashing at greater than the uppermost extreme of 5.8 to 5.9 pH as measured at room temperature risks unleashing harsh tannin extraction from the hulls of the grists various malts and unmalted grains.
In summary, the objectives for acidification (which at this juncture have grown from two to four) are:
1) Mash in water with Alkalinity sufficiently addressed via acidification whereby to not exceed around 5.85 pH (as measured at room temperature) to avoid tannin extraction.
2) If there is any concern for maintaining the very lightest possible final beer color, acidify early (meaning pre-mash) whereby to mash at ~5.4 pH (as measured at mash temperature), or 5.6 pH (as measured at room temperature).
3) Maximize hot and cold break during the boil, and also aid the yeast in reducing the beers final beer pH to the ideal stability range by targeting the achievement of ~5.2 pH by the juncture of hot break.
4) Acidify to accomplish #3 above early (meaning here pre-boil) if maximizing hop smoothness is the goal. Or acidify as late as 10 remaining minutes in the boil to both maximize bold/harsh hop expression and hop utilization efficiency while still allowing for maximum hot break and cold break.
In passing, the present day popularity of targeting 5.4 pH (as measured at room temperature) during the mash and reducing acidification to one single addition step offers a compromise which greatly presumes a nominal drop across the boil of about 0.2 pH points whereby to achieve the real and ultimate goal of 5.2 pH at the juncture of the hot break. But the degree of measured pH drop across the boil has proven to be highly variable, and not an assuredly fixed quantity. So the compromise is a bit of a gamble. And the very least measured drop in pH across the boil corresponds to the lowest levels of added calcium pre or at the mash stage.