As I see it there are several approaches, each with pluses and minuses.
1) If you add acid immediately pre-boil you can benefit from mashing a bit higher, as well as from better fining performance (and/or lessened need for fining as to quantity being added), and thereby achieving a better hot-break, while not suffering from inducing excessive Maillard reaction related consequences and thus having your beers color go too dark on you across the boil, but the drawback is that you will need to add somewhat more weight of hops since hops do their most efficient job of isomerizing at higher pH's. The higher the boil pH the higher the IBU's.
2) Much peer reviewed advice from masters of yore was to first control mash pH sufficiently enough whereby to somewhat (at least) suppress Maillard reactions impact upon darkening the beers color during the boil, and then acidify with 10 minutes remaining in the boil, just prior to adding fining (Irish Moss/Whirlfloc) whereby to benefit from hot break via fining but not require the adding of any additional weight of hops.
3) If you acidify fully post the boil and just prior to entering the fermentation phase you enter thereby an initial fermentation pH range of benefit to the yeast and of benefit in hitting a post ferment pH that resists infection and promotes longer term storage stability (as for the above methods), but you both miss out on the best possible hot break and you permit more color darkening Maillard reactions to run rampant across the boil.
4) If you don't acidify at all pre or during or post boil you potentially miss out on any and all potential benefits of such acidification.
In the end each of us must independently decide our best course of action. I prefer to mash at a bit higher target of 5.5 to 5.6 (call it 5.55) and then add acid to a target of 5.15 pH just prior to the boil along with adding very slightly increased hop additions (if I think about it). The commercials can't afford the extra hops so they trend toward option 2. Most amateurs and homebrewers are at choice #4. They are crossing their fingers and hoping. If you chose to cross your fingers and hope, it may be of benefit to target ~ 5.2 to 5.3 pH within the mash.