Hi Ken,
Rainwater is slightly acidic, so is considered 'soft' and is good for plants that need acidity in their subtrate or to counter high alkalinity. Tap water is also soft but it contains significant amounts of chlorines and minerals which can have negative impact on plant health in the long term. But not saying rain water is 100% good it depends on the area where you live sometimes rainwater is full of chemicals which can also affect plants that are chemical sensitive.
As for aging water, I normally leave it for at least overnight to make sure much of the metal particles like iron settle at the bottom and chlorine has evaporated, most of it.
This is how I do it. So this is my two cents...
