Depends on where you live, I wouldn't recommend it unless you live near the coast, green house gasses such as nitrogen dioxide, sulphur dioxide, carbon dioxide etc all reduce the pH as they create acids when dissolved in water, however, nitric acid can also oxidize to nitrate (Both nitrate and nitrite) which isn't great.
RO water mixed with tap water reduces the KH+GH if you use a 50%/50% solution of tap water and RO water you wont need to add minerals back in as sufficient minerals should be present in the tap water, water with a lower KH+GH is more easily buffered by acids such as humic acid, if you add bog wood then your PH will be reduced.
What species do you specifically want to keep? If it's not Discus, Blue Rams or another species that prefers more acidic conditions but is also quite sensitive then there may be no need to adjust the pH, a lot of species are now tank bred in water with parameters similar to those of the water in our tank.
You could use a pH reducer such as Sulphuric acid but your preparation techniques must be good to ensure that you don't experience pH fluctuations.
But I still maintain that the easiest and most simple way to reduce pH is to use bags filled with peat similar to tea bags, they will stain the water however which is a good thing as it further replicates the Amazon.
There's no need to dechlorinate rain water.
