You should also use your heater in Summer. I know it sounds funny but stick with me on this... The dip in temp at night to 50's from 90s during the day can cause too much of a thermal change during the night and you can counter act this temp change by heating the water at night.

Day--->>> Night----->>> Day--->>>Night
84 ...............78................84............... 78 6 degree difference
84 .............. 82(h)............84...............82 2 degree difference

Basically heat at night to keep stability in the water temp. Temp difference can be the leading stressor in vertebrates (fish).

I hightly recomend atleast one heater if not 2 and ALWAYS on a GFI. This is mandatory for aquariums on out side walls or close to windows. I seperate pumps, heaters and light on seperate circuits. I also run a battery back up on a single power head as a caution.

In general.. we all know that water quality is most important aspect of aquariums... 3 main aspects, pH, salinity and Temp. pH and salinity are affected by Temp. Stabilize your temperature.