Should I be targeting different pHs for different beers?
I can only really give you my process, with the caveat that there is a lot of inference from research, so the accuracy of my assumptions is questionable.
I am continually seeking to control those things that I can truly control in the brewhouse, and mash pH is a small tweak that I can use to nuance a beer, hopefully for the better. Obviously recipe, mash regimen, water makeup, proper fermentation control and beer handling are far more important than minor pH shifts.
First, nothing wrong with 5.4 as a target and that is where I prefer my hoppier beers. Martin has posted data points that imply a stronger hop expression in the kettle at slightly higher boil pH - and mash pH translates into the boil kettle, where it is reduced. I have seen more narrow ranges than 5.2-5.8, like 5.2-5.4, and since I have been able to control mash pH reasonably - been pushing mash pH up or down based on the beer style - and where I can find a data point that might justify it. I build from RO - so this process is fairly straightforward from a blank slate.
For example, I tend to mash wits/weisens and saisons targeting 5.2 mash pH, and that seems to enhance the sourness/acidity I expect in those styles. Wits might get a touch of calcium chloride where the saisons might get a little gypsum. A roastier beer, like a porter, I might push up to 5.5 to enhance the roast bitterness, where a milk stout might fall at 5.3 or so. It just depends. I will try to think about the style's region, look at the water profile and try to determine how the water is treated - and seek out some beers that will be similar to my recipe. I use Bru'n to model the additions and look at driving mash pH in a couple of directions, and think through what my goal is for a given beer.
I may brew couple of 1 gallon BIAB test batches and try to refine the mineral profile and recipe if needed before committing to a full 11 gallon volume brew. It's a handy way to evaluate side by side, and makes a great starter for larger batches. I very much enjoy this process and welcome the practice. I should add that I only submit to competition a few times a year - and while my scores have improved a lot, I don't have enough BJCP scores or wins that prove this makes a lick of difference, just the increased approval of my friends (with a few professional brewers and judges) and my own palate.