For me, basic water treatment is about fixing mash pH and good enzyme activity. So I only treat strike water/mash liquor. Untreated hot tap water for sparging. I can't say I've noticed a difference with full-volume mashing, when total water gets treatment. All salts, including CaCO3 (chalk), dissolve readily in the mash liquor, but not so well in water. I think I just find it an unnecessary chore to treat two volumes of water. A little calcium (~1/2 tsp, CaCl2 or CaSO4, for a standard batch) added to the boil is sometimes a good idea, if you're not happy hot break or fermentation performance, especially yeast flocculation.