I agree that it really depends on YOUR tap water. There is no right or wrong answer. It's like asking if you should bring an unmbrella to work today....depends on the weather where you work!
My tap water is extremely hard (23 grains, practically crunchy). I buy 5 gallon jugs of RO/DI water at a local pet store for $3.25 a jug. Through experimentation I've found the best mix of RO/DI and filtered tap water for each style. For example, for IPA's I do 50/50. For Lagers or Pilsners I do 90/10. For pale ales and wheats 75/25 (RO/DI to tap ratio) etc. etc. I played around with measuring water chemistry at the different mixes and fine tuned through experimenting and now, without a lot of fuss, I get a pretty good water profile for each style. I also use PH5.2 in my mash so I'm probably not optimizing my mash PH but my beers turn out well so I'm sticking with what works.
I do all grain. Haven't tried this method with extract or partial boils.
Conical : bohemian pils
Keg 1: Brown Ale__Keg 2: Hellas
Keg 3: Hellas__Keg 4: air
Keg 5: AIR 6: Edworts Apflewien
My HERMS rig