I would not use rain water personally but your geographical location will play a big part as well. Yes, you could filter it into usable water but when its all said and done then you might as well just pay for the water instead of the filters. Look at it this way. All the pollutants and nasty stuff put out by cars, factories, planes, etc linger around in the air and is absorbed into the clouds and falling rain. Think about acid rain for example, don't wanna put that in my fish tanks
.. you could however incorporate a nutrient export system on your tank with some house hold plants or aquatic plants prolonging water changes which means less water being used and less family bickering lol.