I would never leave a heater unplugged. You simply just don't know when the temperature is going to drop. A heater ensures a stable temp, even in the summer. My fan turns on at 78.0 degrees (chiller on at 78.3 if needed), but my heater turns on at 77.5 so my tank never fluctuates more than 0.5 degrees. Stability is the name of the game in a hobby like this.