Well, my tap water is chloramine treated, so I cannot test your out-gassing theory BillD, so I can't support or refute it. If it works for you, great. But for the novice fish keeper, the Advisor's are going to recommend tried and true technics...which means ALWAYS treating tap water with dechlorinator.
As for 'enough being left over from last time to deal with 20% new water'...I'm not a chemist so I don't know how long the chemicals in a dechlor stay active...AND if you're not adding extra in the first place, how could you have extra left over a week later to deal with 20% more water? Plus you're removing 20% of the left over dechlorinator during the water change.
Because these issues don't sit well with me, I'll continue to stick with my recommendation.
I suggest Prime because of its high concentration and effectivness.
When doing a bucket brigade, I add .2mL to each of my buckets (as 1mL of Prime treats ten gallons), and then fill them with proper temperature tap water. Thus by the time I've filled the last bucket (I have 3), the first should be plenty safe to dump into the tank.