The tank will NOT go up to a 100F. It would have to be very very close to a heat source such as an electric heater or heating duct to get that high.
I have 2 x 25w in a 15g gallon and they cannot keep the temperature at even 78F which is there preset value.
I would recommend you get the heater,
If you have a thermometer, find a container and add slightly warm water, using the thermometer, record the temperature until it gets to 78F. This will give you an idea of the "feel" of the water.
That way you can manually check the tank and see if it "feels" too warm or cold. (using the thermometer as well)
I have used this method for small tanks which I absolutely only use in a TOTAL emergency until I can short the problem out...never had a fish die and a few degrees temperature + or - will not affect them...water temperature in nature rarely stays stable, there is change due to rain and sunny days etc.
5-10g tanks are not very expensive and it would possibly be in your benefit (budget allowing) to look at getting one of these. It gives you a more stable environment for the fish than a small 1g tank would.
Hope this helps.