You will get many different answers, and several different explanations. So I would suggest that which ever temp you decide to shoot for, try to keep your temp at that level. Don't let you tank temp swing back and forth from day to day. If you try to keep your tank at 77 degrees, and you have day where it's a 77 and Sunday, 81 on monday, 79 Tuesday, 83 on Wensday, It would be better to set your heater to keep your tank at 81 or 82. This is the temp your tank basically runs at. and it would be much simpler to keep the temp at that level than fight to keep it lower.
I personally keep my tanks at 82, My reasoning for this is because 90% of the fish are caught in shallow reefs. the temp of these waters are closer to 82 than 78. But as I said you will get many different suggestions. Find what you feel comfortable with and keep your tank stable at that temp.