I've kept goldfish now for about 5-6 years in somewhat elevated temperatures allowing them to be kept with non-cold water fish. The key thing though is to keep the water stable at those higher temperatures. I have a lot of lights for plants with CO2 injection so they generate a lot of heat so I did put a chiller on it because in the summer the temperature goes up too high for this elevated temperature and the goldfish.
I've essentially locked my tank into 76-77.5 degrees and it regulates with the lights pushing heat up as well as a heater that kicks in at around 75 to go back to 76 on the digital meter on the chiller at worst case. The chiller turns on once it goes past 77.5 degrees and pushes it down to 76.5. The key thing with goldfish at higher then 72 is you need to be sure to deal with all signs of disease immediately. I change the water very frequently as well as have a UV sterlizer on the tank.
It all depends on your local area and room temperature. In California where I am it gets rather hot in the room I have the tanks in so it needs the chiller or I would end up with 80+ degree water. Before we had AC installed my girlfriend lost her goldfish on a rather hot day while she was at work. When she got home the tank temperature had risen to 87 degrees by itself. She had unplugged the heater for the summer so this was not a faulty heater. Even with my AC in the summer without a chiller it will go to around 82 in my planted tank.