Natural reefs get swings in temperature, some quite large. Lots of studies going on now due to the climate change interest. The thinking backed with research is that coral get used to swings over time in nature by adopting clades of zooxanthelae that are more suitied to different temps. Some are adamant about adapting your system to cope with temp variation. The idea is that a tank with a very small daily swing or constant temp is less able to take a spike of even a few degrees than one where the temp varies regularly. Not advocating this but it's worth noting for those who may want to try it in situations where constant temp is unatainable as a practical matter.
Personally,I don't fret over a swing of a few degrees but almost never go over 80. I think my whole sytem would crash over 85 though. Fortunately ,I have central air andi n my neck ofthe woods more importantly a good furnace and a generator to cover power outages.
When temp goes up metabolic rates go up, O2consumption increases and the warmer water holds less soluble oxygen. Zooxanthelae go into overdrive , producing more oxygen than the coral can handle( oxygen radicals) and/or actually competing with the coral for food leading to expulsion of the zooxanthelae by the coral, known as bleaching.
