For many fish, if you are changing around 20-25% of the water, the new water should be close, but it doesn't have to be exactly the same temp. I filter water into a tub, and it is usually around 65F when I do my changes. The tank is around 76-78F, and I have never had a problem. The overall temp of the tank only drops 2-3 degrees, which isn't a big deal for most fish to handle--heck, for some species, it initiates spawning!
For sensitive fish, you can use mix hot and cold waters if you fill straight from the tap. If you use filtered water, or allow your water to sit in a tub overnight to gas off chlorine, just use a small heater.