Right out of the tap for me as long as the PH is good, and I've never seen anything but neutral. If you are worried about your water source then you are probably making mistakes in other areas of the grow. I doubt any plant has ever died from tap water. My favorite nursery uses tap water right from the hose I see them watering all the time. Their plants are flawless.
On the other hand bottled water is evil. If every truck that ships water parked for good tomorrow it would save billions of barrels of oil a year. You pay more for bottled water than gas, and you pay more for gas because of bottled water.
Water is shipped from your state to states far away, and from states far away water is shipped to yours all for the sake of providing people with their favorite label. It's the equivalent of a 5 year old deciding what Disney character to like. There are a lot of stupid people in the world. There is no better evidence of this than bottled water.