I use water from teh water butts when rinsing my car off after washing, no streaks and no water marks, it has a built in filter between the downpipe and water butt, the filter needs cleaning out about once every 3 months as it does collect some sand (Live 150yds from beach) and other debris.
No worries rinsing my car with water from it at all, regards "Acid Rain" is it any worse for your car that it sitting the rain anyway?
, it wont be an issue unless your car NEVER goes out in the rain or is permanently in the garage (in which case why bother having a car)