Having studied greenhouse emmissions from power generation at university, I cannot say clearly enough NO.
There is so much you don't want in your tank. If you live anywhere near a highway you'll find in your air CO, SO2, NOx, Pb, CFCs, O3, any combination of polycyclic aromatic hydrocarbons, 1,3 butadiene, benzene, whatever else is in photochemical smog, and if you live anywhere near a coal fire power station there'll likely be dust and flyash too. This will all come down onto your roof both in the air and in an aqueous solution in your rain.
Tap water, provided it doesn't come through too many copper pipes, is much better IMO