Mashing at a room-temp pH in the lower end of the range (say ~5.2), does invite excessive proteolysis and the resulting beer can be a bit thinner. If you're dealing with a big beer and need to reduce body, then that lower pH target may not be a problem. For most other batches, I would not go that low for the mash. 5.4 is a pretty good target.
As you've probably heard me mention, there are beer styles that benefit from having a lower wort pH. But that can be achieved after the mash via an acid addition during or after the boil. German brewing practice does target a more normal mashing pH in the 5.4 to 5.6 range and the wort is acidified at the end of the boil to bring it into the 5.1 to 5.2 range. The higher pH helps convert and expel DMS from the wort. The pH reduction helps the yeast out and ultimately provides a crisper flavor presentation in the beer. Since I haven't heard of British brewers conducting post boil or pre-ferment acidification, I'm not too inclined that they mash at too high of a pH. But the other consideration is that typical British ale yeasts do have decent acidification capability.
To those that want to give high mash pH a try, I say go for it. But my experience is that high mashing and wort pH is the number one cause of the dull beers that we often find from homebrewers and craftbrewers that don't attend to water chemistry adjustment.
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