I would not do this. I fill my tanks directly from the tap, I run both hot and cold and get the temp close to the aquarium (by hand testing) and then switch the valve and it fills the tank. I squirt in the conditioner as it starts. This requires a "Python" implement, but the point is that I do not let the tap water sit. Bacteria is not the issue, as someone mentioned. I have heard of people not using the hot water, just the cold, for other different reasons.
With a 40g, doing say a 1/3 water change, the easiest method is to take a small container of water from the tank to the sink, run the water until it is close to the same temp as the water in the container, fill a pail adding the conditioner, then pour or siphon the water from the pail into the tank.