Well...I use a 50 gallon trash can on a dolly for my water changes (have no choice) so I add the conditioner for the volume I'm adding as I'm filling the can, then into the tank it goes, thoroughly mixed and pre-treated.
I also pre-treat my water before adding it to my tanks. De-chlor works fast but why chance it. Sure I am lazy sometimes and have done this stunt but I would never do it to any of my prized fish. It isn't worth it. If you must take the lazy way, add the de-chlor to the tank first and add extra. Never add the water then the de-chlor. Just because it doesn't kill them does not mean it does not affect them.