My house gets cooler in the fall. Do my ants need a heater, or are they fine at room temperature?
Yes, most ants will live at room temperature. They'll just grow more slowly, since room temperature is below the ideal for the vast majority of species.
There are times where certain species do better without heat. Species that diapause need their heat taken away for the winter, so cooler ambient conditions in the fall are actually a good thing for Lasius, Formica, northern Camponotus, etc. Fungus-Farming Ants are best left at room temperature, since heating cables can cause condensation that's dangerous to both the ants and their fungus.
If you do add heat, use a Heating Cable on a thermostat and only heat one side of the nest. Your ants need a cool side to retreat to; if it gets too hot, the entire colony can perish.
How To Heat An Ant Colony guides you through the setup.