A 5 degree swing is peanuts once the fish get used to it, and you keep your tank maintenance sharp.
With nanos, you have less volume as a buffer against nutrient-dump disasters: you know, something dies, and the rot not only unleashes ammonia but hogs all the oxygen in the water?
Keeping the water on the cool side not only slightly increases the amount of O2 against such a disaster, but keeps animal metabolisms a bit slower so they need less O2 to begin with. I'd probably shoot for 78F with an acceptable swing of 2 degrees either way.
I dealt with excess daytime heat via evaporative cooling in the day, keeping a fan on the water surface and regularly topping off with FW what evaporated.
If I'd had to deal with excess cold, well, maybe I would have gone for a heater ---but only a brand I could trust not go nuts and turn my nano into soup.
