My experience-
I have not used a heater in over a year. I only did this after paying close attention to my heater, seeing how often it came on (almost never), and the ambient temp of my house. Using common sense, I deducted that the very gradual cooling and rising temp in a 55g *shouldn't* have any ill effects. I pay close attention to my fish, and have learned when they are stressed. In the heaterless last year, I have not lost a single one, and they are all thriving.
PS- I live in Socal, where the nightime lows are never below 30. So make sure to take that into account.