In the end, what is going to affect flavor is the pH of the finished beer, not the mash pH or boil pH. There are several factors that affect what happens between the time you measure your pH in the mash and the time you drink the finished beer. Concentration in the boil, yeast activity and carbonation are all major players.
Having said that, I generally try to target a mash pH and let it ride from there. That is mainly for simplicity's sake - I like to make all my adjustments in my strike water so I don't have to worry about making (or forgetting) adjustments later on down the line. If I feel like I need to make a pH adjustment after tasting a recipe, I will change my water treatment the next time I brew the beer.
Here's what I generally shoot for with mash pH (@room temp):
5.2 - Saison & sours
5.3 - Lagers
5.4 - Hoppy ales, and most malty ones (going to try my Brown Ale at 5.3 next time around as a test)
5.6 - Stouts & porters (I don't currently brew a dry stout, but that would be an exception)
I feel that these are good starting points to get me in the ballpark of what I'm looking for. Like everything, I think the best approach when fine-tuning your brewing is to have a consistent procedure, then tweak one thing at a time until you hit your target. pH is another tool in the toolbox, but I think of it as a specialty tool that I only reach for in specific circumstances.