I personally wouldn't, since I think the soaps and chemical residues are pollutants. Others may disagree. I suppose it depends on the need. I guess I could see that being practical and perhaps necessary in areas like Nevada or California, where people have created a virtually unsustainable system composed of far too many people and far too little fresh water. In areas like the great lakes, where we may not have balmy weather year around and tons of sun for solar farms, we do have plenty of water, and it makes much better sense to treat both "black" and "grey" water, rather than polluting our farmland and urban gardens with tons of detergent and fabric softener. I would hope than any such use of a cistern would be regulated by the government, so that a potential buyer of the property in future years would be well aware that the cistern has become toxic.