There has always been and probably will always be a debate on whether the total tank volume vs the amount of water being changed should be treated.
Most dechlorinators have different doses for what you are treating the water for (chlorine/chloramine). If the difference is one or two capfuls, it won't hurt at all.
A great many folks are probably not aware of what element is in their water and it would do good to find out. This info can usually be obtained from your local water supplier. If your water is supplied via a municipal system, it can be found on your local government website.