I keep my heater set on 81. I found that with my lights on, the temperature would sometimes climb up to almost 83 degrees on really hot days. Plus, my wife has this tendency to say "OMG, it's too cold" and go turn the AC off. I don't notice it, the house heats up to the mid 80s, and that's not good for the tank either.
So, it's better to have a steady temperature, than trying to aim for the magic 78 degrees and having it fluctuate due to lighting, outside temp, etc. Keep it a steady 80-82 degrees and you'll be just fine.