I recently implemented a 300 watt heater for a biocube 14 tank. Definitely overpowered. I have heard conflicting things about this. I think an overpowered heater will take less effort to keep your temps stable and less wear and tear.
My temps seems to have stabilized. 78-79 with full lighting. Maybe, its the weather..... But I have had, heat on/lights out, heat off/windows open...whatever configuration you can think off these last few days.
Heater accuracy should definitely be checked. I have three types of thermometers. I have lost fish due to heater issues before. I suspect chillers have redundancies and failsafes unlike a cheap heating coil in a testube.