Tempurature Fluctuations

3 degrees is not a big variation like has been said but if you're not climate controlling your tank for both hot and cold situations you should look into it. All the cold weather in the south US this year has caused some serious problems for some reefers who weren't prepared, for example. You never know when you're going to get a cold snap or a heat wave.
Fans/chillers and a heater are good ideas even if they don't need to turn on very often IMO.

Also, your acceptable temperature swing will depend on what you're planning on keeping, some corals can handle larger swings, and some will probably do best in more stable conditions. Also, there's a difference between what is acceptable and what is optimal, I personally think keeping things stable is better for most coral and fish. Nature is sometimes just acceptable, and not optimal too.