This has always been a very confusing topic to me. I've tried to standardize my practice with things Ive read from Kai, Martin and Gordon among others(thanks guys!). What Ive come up with as my standard is this.
Assuming the bulk of my mash time is spent in the 148-158F range.
I will measure pH at room temperature and adjust about .2 for the pH of the mash.
I typically try to achieve a ph of 5.2 - 5.5 at mash temps. So 5.4-5.7 at room temps.
I do 5.2 for beers that I want to have a bit of twang(heffs, wits)
I do 5.5 for beers that I want malty(stouts, scot beers etc)
5.3-5.4 for everything else.
Not sure if my logic is flawed but at least its something that is consistent from batch to batch and I can adjust from there with acid/pickling lime.