A large water change is required because when you are doing a fishless cycle you are repeatidly dosing ammonia. The nitrates are not removed so they just keep building up. You end up having sky high Nitrates. Before adding new fish to the tank you need to perform a large water change to remove the high levels of nitrates.
Since you are cycling with plants in the tank, test the Nitrates and be sure they are roughly 10 before acclimation. If they are higher, go ahead and do a change. I say 10 because that's a nice low number. If it's 15, nbd.
I do weekly water changes of at least 50%. We do 75% in the 150 Cichlid. Water does not need to age. Simply add a dechlor. The only reason why people used to age water was to remove chlorine. Many water suppliers are using chloramines now. They are not removed through aging so dechlor is required anyway.
Regarding the fish shock, this is a new tank. There will not be fish in the tank to shock. When doing this to an established tank, its not as big of a deal either as long as you are adding fresh water. It's similar to acclimating fish to a new tank. I agree with jcar, my fish actually enjoy swimming through the fresh water when I am syphoning it into the tank. And it's untreated. While filling with the Python I add dechlor but the water going into the tank isn't actually treated yet.
