I can't imagine that the temperature of tropic water around reefs changes much in the winter. Remember, most of the places where we find the corals we keep don't actually have winter. Most places just have storms and, depending on the time of the year, it comes from the left or the right.
The only region of the water that changes significantly in temperature would be surface waters with the changes in rainfall. Yes, some areas deeper down will change with changes in currents but this, at least in my mind, changes less frequently that the weather.
There are a number of papers that show corals that experience regular fluctuations in temperature are actually less susceptible to temperature-related bleaching than corals that have normally stable temperature conditions. So, there may actually be something to varying your temperature on a semi-regular basis.
Historically I have just set my temperature and let it change with the weather (in the summer getting up to 82F and down to 78 in the winter. However, after this summer (where my tank hit 87F during a heat wave) I've now added a chiller and so will probably just be keeping my temp constant now.
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