I do the same as Twistersmom and have for more than 15 years with never a problem except once when I forgot the water conditioner (more in a moment).
Aqueon also make one of these, and in my experience the faucet attachment is much better constructed; my Python faucet attachment broke three times (threads wore out and it wouldn't stay on the faucet--result, flooded laundry room
I attach it to the faucet and turn on the cold water, then syphon water out of the tank (vacuuming parts of the gravel substrate) and drain out half the tank (I have 115g, 90g and 70g tanks). The I take a cup of water from the tank to the faucet, adjust the hot/cold to the same temp by hand, turn the valve, and the tank starts to fill. I squirt one squirt of the water conditioner in front of the hose in the tank, and leave it to fill. On my SE Asian tank I have the incoming water the same temp or slightly higher because of the swamp fish (Chocolate Gouramis, Pygmy sparkling gouramis) that need warmth. But on the Amazonian tanks I have the incoming water slightly cooler to stimulate the fish as it does when it rains in the tropics. The temp only varies by 2-3 degrees F max.
The only time I had a problem was once in 1998. I always sit and check the tank after the pwc, just to ensure filter is running (having shut it off with the heaters obviously) and the fish look OK. That one time I noticed most of the fish gasping at the surface with red extended gills; I instinctively knew I'd forgotten the conditioner. Squirted in several squirts, fish recovered after several minutes, except for a couple that were too far gone. The chlorine/chloramine burns the gills. Now I just sit the bottle of conditioner on the tank light before I start to drain it, and by force of habit I remember.