Bettas are usually more prone to disease at room temp, I'll testify. I made a lot of learning mistakes with bettas and heaters were definitely one. Of sourse, I wasn't using live plants back then and my water has no hardness, so temps weren't my only enemy. So it seems to me, in the place where I live, a heater is certainly a must. At least keep them above 75ºF.
Also remember that some people have nicely sealed homes that are easy to heat and stay at a constant temp. My house was built in the 1920's and goes from 50ºF at night up to 82ºF by day. Another factor that made heaters a must for me.
Do you have a larger aquarium you can put the betta tank on top of? You could use the radiant heat to warm the betta.
They do sell teeny submersible heating pads that simply put out 5 watts all the time. The one I had was not adjustable and was hardwired to always be on. Great for winter but I had to unplug it in the summer. The element broke one day when I was doing a water change. Fragile things, they are.
I'd just upgrade to at least a 3 gallon tank/container and get the smallest glass heater that fits the bill.
