I still don't understand how making a dosing solution is a way of making things simpler. Adding extra steps means adding complexity in my way of thinking. Roughly measuring out 2 or 3 spoonfuls a day and tossing them in the aquarium only takes a few seconds.
I do make solution for micros however, just because the dosage for anything less than a very big tank is such a tiny amount.
I like using distilled water when mixing hydroponic fertilizer concentrates just because calcium salts dissolve better than in my hard, local water. They're always in a crystallized pentahydrate form which probably makes them less soluble.
Even hydro nutrients could be dosed dry, but I premix simply because I'm copying commercial hydroponic fertilizer mixes.
I dose all the macros dry in my aquariums so I can't really say I've tried salts like KNO3 in tap water. They dissolve pretty well in my aquarium though, so I'd think tap water should be fine.
I use distilled water for mixing CSM+B also, but honestly I've never tried tap water, so maybe it's just a waste of good distilled water.