My aquarium is 2 gallons so it's small. I do have a thermometer and so far the water temp has not gone below 70* (it's been around 73'ish). The room that the fish is in, is a very warm room so I don't want to make it too hot for her.
According to the book "Bettas" by Marshall E. Ostrow published in 1980 "An aquarium constantly maintained at about 80 degrees F. is a bit too warm for most hobby fishes but not for bettas. In fact, 80 degrees F is just about ideal for most bettas."