It really depends on the area you live in,If you live somewhere out of the way,the water is less likely to be filled with pollutants rain water is striped of heavy impurity's due to evaporation.You would need to have a clean collection system and of course test water samples to see if the params check out ok. A friend of mine uses a funnel 2litre bottle to collect rainwater for his betta,and he has never been in better shape,I collect it for my reptiles and they are in top shape aswell.And if i had the material to make a large enough collection system i would probably use it for my WC's.
Just my Experiences and opinions,
-AC
[EDIT:Gmh is correct always collect it BEFORE it hits the ground.]