78 is good for a betta. Anything below 75 in my experience has made the betta lethargic. I keep my betta tank at 77-79°F. I don't know the celsius equivalent.
As winter approaches, a heater might be needed. I live in Florida in the US. Right about now it is starting to get colder so I've had to turn my heaters on. If it is always above 80-85°F where you live. then you can get away without a heater
I live in Mh, India and the temperature here is always in the range of 20-30 deg C outdoors and normally above 25 deg indoors during most of the year, anyways I'll keep a heater just in case, better be safe than sorry