most fish keepers I know over the last 25 years only dose for volume of new water.
this 'dose for full tank volume' only became popular since seachem prime began gaining exposure with the rise of the internet sites.
you definitely don't need to dose full volume for any dechlor.
your fish will be fine.
the old water in the tank would have been fully neutralized from the last dose, so adding extra to 'treat' the old water that was already treated prior doesn't help.
its a waste of water conditioner, and overdosing with any dechlor can lead to cloudy water in some cases (a precipitate formed)
however, this is pretty rare. you have to add lots too much to do this.
if you are dosing for full volume and seeing no ill effects, and you feel more comfortable with it, continue.
but if you aren't, there is no real reason to start dosing (unless, as mentioned earlier, you are using Prime that helps with ammo/nitrite) and your tank has an issue.
in established tanks, plain old dechlorinator (something with sodium thiosulphate in it) is all that is needed.
good discussion though.