If you know that the water you use to wash/detail is hard to enough of a degree to leave water spots as soon as it evaporates then I would look into some sort of softner or filtration system. As far a water for dilluting products I would just use filtered water unless you fell the cost of distilled water is worth the peace of mind.
I personally have a whole house filter that gives me RO water to all but one tap. It is nice not having hard, chlorinated water in the shower and to have to wash vehicles with. Plus I never buy drinking water for the house; it's not the purest drinking water however, it's way better than the city tap water.