Generally the lights will add enough heat to the tank, in fact usually they add too much.
Your heater will come in handy at night when the lights are off.
I assume you don't keep your house at 78 -80 degrees in the winter.
If you keep your thermostat at 68, your tank could drop a few degrees during the night then heat back up during the day.
Small shifts in temp are normal (sunrise to sunset), but you really want to keep your temp as stable as possible.
I, on the other hand, have 3 large pumps on my 250 that keep the water at 79 degrees for me! No heater necessary for me!
For a smaller tank, I would recommend keeping the heater to keep your temp stable as possible.
Good Luck!