If it's working for you then fine, but if all you are trying to do is soften the water and lower the PH. You could put some dried oak leaves in the tank while adding some driftwood. The only good thing that comes from rainwater is probably the mosquito eggs, which are good source of protein for the fish.
If you have an established tank (6 months or longer) you can use regular tap water and leverage a sprayer in the sink!
It's a huge (often accepted) myth that beneficial bacteria only lives in the filter when in fact there is far more in the substrate... Filtration and Water Quality
A caution about rainwater ...
I live in the Northeastern United States and we have what's been referred to as acid rain. In many places these days there is enough air pollution to contaminate rain water, the degree of which might be debated. I also at one point attempted to collect rain water from my asphalt shingled roof. I live in out the country, but have a fair amount of road traffic. I quickly noticed that the water was tinted yellow and clearly not suitable for aquarium use.
So rain water may or may not be a good option. A much safer approach to cut hard water or for other reasons would be to use RO or distilled water. And again, for filter sponge cleaning, tap water can be just fine.
Note: If you live in the wilderness and have a metal roof, rain water collection may be just fine!