Hello,
I used to sell fish and my personal opinion is that having a heater and not needing it is a whole lot better than needing it but not having it. No less, another major factor is the carrying capacity of water in that the major (one of them) argument for heating an aquarium is to keep the temperature from swinging up and down throughout the course of the day. Subsequently, if you can keep the water temperature constant in and ofitself, then a heater is not needed. The problem with this thinking is that only very large aquariums have enough mass to retain heat well. Unless your tank is larger than 50 gallons, you are not likely to have a big enough tank to prevent swings in temperature.
One side note: You should be aware that the recommended tank size for 2 fancy goldfish is at least a 20-gallon aquarium. Regular goldfish fair best in tanks with a capacity of 40 gallons or more. If your tank is not of the sizes, then you may want to consider other temperate fish.
MOA
P.S., I'm not saying the fish will suddenly explode if you keep them in too small of a tank, but be aware that most goldfish in tank under 30 gallons in capacity do not live longer than 5 years. The record was around 50 years, but that was achieved in a tank almost 50 gallons in capacity with a relatively small species of goldfish (many goldies can grow over a foot long if well cared for).