There are two basic reasons for mineral salt additions.
The first is to help control the mash pH, in which case the minerals should all be added to the mash to buffer the pH in the desired range. Adding the minerals to the sparge water does not help with controlling the mash pH.
The second is for flavor, in which case they can all be added after the mash so as not to affect the mash pH.
For some recipes, I split the mineral additions so that the critical salts for controlling the pH are added in the mash, but those added for flavoring (Carbonates, high levels Sulfates or Chlorides) I add to the collected wort and before the boil.