PH during the mash is the most important pH. It should be between 5.2-5.7, measured at room temperature. As you boil, the pH should drop. During fermentation the pH will drop more still. You don't really need to worry about the post-boil or the post-ferment pH.
Your sparge water should be under 6.0. I usually add a tiny bit of acid to get the sparge pH in that range.
I use phosphoric acid to lower mash pH. Most of the time you will probably need to lower you mash pH. If I need to raise it (which is hardly ever) I use slaked lime.