I'm curious as to whether it's entirely safe to introduce chlorinated water directly from a tap at any point at all.
But I assume that the reason the whole volume is treated, is because the dechlorinator is naturally going to dilute in the whole volume of the water in which it's introduced.
In other words, if you have 10g and you're changing out 5g, and you only add enough dechlorinator for the 5g - once the water volume reaches a total of 10g the dechlorinator will then be at half strength.
Then again if the ppm of chlorine in the total water volume is only half as much, you might only need half the amount of dechlorinator to neutralize it...so I'm confused as well. But I'll say this - if I'm taking a risk as crazy as adding any amount of fresh, unaged, chlorine or chloramine-laden tap water directly to my tank, you'd better believe I would dose that system at full volume strength.