I've seen this discussion debated (and I've partaken) so many times on many a forum...
There are proponents of before/after, total volume/adjusted volume and even whether or not conditioner is really needed in most cases where only chlorine is used.
There are some studies showing that the average chlorine dosage is so minimal as to be of little/no impact if water changes are less than a certain percantage, 25-50% +/- (?)(don't recall numbers).
But disregard all of the debate/discussion...
Just think this way, "Better safe than sorry."
Why take a chance that for five or more minutes (5+min) chlorine/chloramine may be killing beneficial bacteria colonies/burning your prized fish's delicate gill membranes while you refill...
What if your distracted away with just enough time to turn off the water?
Dechlor added before hand reacts almost instaneously to detoxify chlorine/chloramine, virtually no chance of anything harmful happening thuswise.