With a smaller tank you could get away with putting tap water straight into a tank then treating it. Yours probably takes around one 5g bucket tops and very little time but then again why not treat the 5g bucket of water first. Once you get a bigger tank and you are adding multiple buckets and it's taking longer to do a water change then I would add your dechlorinator to each bucket as you go. It's easy enough to break down the dosage amount and if you are putting a slight amount more than you have to it won't harm anything.
IMO, I wouldn't dump untreated tap water into a tank that takes me 20-30 minutes or longer to fill before I'm done.....though some people do that. If you use a Python siphon to fill your tank make sure you get the temp of your tap water within 2 degrees +/- your running tank temp. before hooking it up and filling it in your tank. Temp swings can harm or kill fish. Again, depending on the size of the tank, you are adding untreated tap water to your tank for several minutes. Some people will change out the water, dose the tank, then begin siphoning water into the tank... allowing the water to begin treatment inside the tank as it's filling. I personally like to treat the water before adding it to a tank. Another reason I personally don't do this is because tap water generally has chlorine and chloramine....mine does as well....that can kill the beneficial bacteria in your tank and harm your fish. I don't like to chance dumping it directly into the tank untreated. Fluctuating temperature is also something to consider with a Python siphon fill.