It really all depends on how much electricity you are willing to use to keep your house cool, and where your tank runs vs the ambient temperature. I let my house get to 79-80 during the summer which is not cool enough for my aquarium to stay at the proper temperature. Otherwise you can be doing a tap dance right at the tipping point for disaster.
That being said, I use a chiller here in Redding, set on 79 degrees. I leave it online year round. Over night in the winter when the house is 69 degrees, my tank will get no cooler than 77 degrees, and without the chiller during the photoperiod my tank will hit the low 80's even in the winter when the house is 72-74.
It might kick on once or twice a day in the winter, more often in the summer. For me, it is both insurance against a house A/C failure, and to allow me to keep the house a little warmer - and I never give temperature a second thought..
When I was running LEDs I went without a chiller for a while and even then, there was always that stress when I would come home on a 85 degree day and the family had the A/C off and all the windows open - house at 80+ degrees with a fan on. Yep, saves electricity but jeez, its tiresome worrying about the tank hitting that magic meltdown temperature.