On the latest broadcast of Brew Strong, John Palmer suggests to add your total salts in two additions:
1) during the mash, and
2) during the boil
His reasoning/process (as i understand it) is this:
Figure out the TOTAL salts you need for a target, then split the addition in two.
1) add enough salts during the mash to get your pH, Residual Alkalinity, etc. in line for a decent conversion, then
2) add the remaining salts to make up the difference to reach your target to the boil in order to hit the right flavor profile. Specifically Cl:SO4 ratio.
Palmer mentions adding salts to the bulk water before doing anything, nor adding salts to sparge water won't be as beneficial because of the relative insolubility of most of the salts.
For what that's worth...