You'll get a lot of varied responses on the use of 5.2. Some love it, others see no point. It depends on your water. I think those who love it, have water that is close, but needs a little tweaking. For those with fairly hard water, this product is not so effective as it would take so much that you would taste it - or you end up adding so little it doesn't perform it's advertised job.
If you want to use it, the mash is the main place to add it. If you batch sparge, you might not need to add it to your sparge water. If you fly sparge, then you might want to add it. Again this will also depend on your water. If it has high pH and alkalinity, then you might see more benefit to using it (in sparge water).
I personally add it to both, but this is simply to use it up. I treat my water to reduce it's high carbonates. This requires me to then go back and drop the pH down to the proper range so I toss in a little 5.2, and then use phosphoric acid to reach my target pH. Again this is simply to use it up. I could simply use the phosphoric acid, and will do so once the 5.2 is used up