It depends on where you live. Here in the northeast, there's no way my fish would survive without a heater.
I have 25w heaters in all of my 2.5 gallon betta tanks. Unless your room temp. stays at about 77 - 80F, your fish will be too cold - they're tropical fish and it's amazing how many myths about what they don't need are out there.
Anyway, It would be really hard to heat a 1 gallon tank. Those mini heaters only heat the water one or two degrees above room temp. so again, it depends on where you're living and how cold the room temp. will get.
You can always get a critter keeper. The large ones are almost 3 gallons and sell for $7 or $8 around here. You can put a heater in those no problem.