Either way will work, but I recommend allowing fluctuations.
Conventional wisdom in the hobby is that you should minimize stress by avoiding temperature swings. The trouble with that logic is that there is NO evidence that temperature swings are stressful to reef inhabitants in the first place. In fact, all of the evidence suggests that at least in corals, fluctuating temperatures induce physiological changes that make them hardier and less susceptible to temperature stress. Corals from fluctuating environments even have higher photosynthetic efficiency than corals from stable environments.
To paraphrase Stephen Coles from his 1975 paper- the maximum temperature matters, not the fluctuation. The stress threshold for corals is 2-4 deg F above the average maximum temperature they've been acclimatized to, whether that's 78 or 86. They can tolerate 10+ degrees of fluctuation within a few minutes without any stress (provided they've been acclimatized to that range), but if you exceed their normal maximum temp by as little as 2 degrees, that could cause bleaching.
Anywhere between 76 and 86 is fine. No part of that range is better than another. It's possible to go slightly hotter or slightly colder, but it's riskier and there's no benefit to it.
80 is far from the high side. More than 2/3 of the world's reefs, including virtually all in the center of reef diversity, have average temps greater than 80. The worldwide average is 82.
