Lets say that hypothetically speaking you only take out a gallon or two from a 10 gallon tank, when you add the new water, do you still use dechlorinator? Since its so little do you really need to add some?
If chlorine is present in the tap water, then dechlorinator should be added to any replacement water. Most probably have significant levels of chlorine in their tap water and this is highly toxic to fish.
Some areas use very little chlorine, and you can get away with this. But the day can come when the municipality decides to increase the chlorine to deal with "x" and it could be game over for the fish.
The other thing here is that changing so minimal an amount of water is basically pointless anyway. Every W/C has to be substantial in order to do what it needs to do, remove pollutants that weaken fish and add fresh water.