While i do agree on the whole stability thing, i find it kind of hard to follow as far as temperatures go. If you're in the wild, you will see temperature fluctuations all the time, depending on the weather. If the sun isn't out, and it's raining over the ocean and there is a cool breeze, the water WILL be a lower temperature, while still staying warm though. If there isn't a cloud in the sky, and the sun is beating down on the ocean, the temperature is undoubtedly going to be higher then when there was no sun out. I mean.. these are all factors we don't really have to take in to consideration, since we make our tanks experience whatever type of weather we want.. but i think it's normal for your water temperature to vary between 78-82. At night, my water temp will go down to 79 and during the day, after my light has been on for a couple hours, it will rise to 81, but never raises any higher than that. Having stability in the tank is always a plus, but when it comes to temperature, as long as your within some sensible range i think you can be a little more lenient.
Just my thought on this subject.