There is no magic number that fits every house and every family, and be suspicious of anyone who insists there is. What there is: ranges that balance comfort and cost for most homes, plus a few habits that matter more than the number itself. For many La Vergne households, cooling set in the 74 to 78 range in summer and heating in the 66 to 70 range in winter is a workable starting point to adjust from, not a rule to obey.
Your house changes what a setting feels like. A shaded Woodland Hills home with good insulation may feel fine at 76 in July, while a west facing home near the lake with big windows may feel warmer at the same number because of sun load and Percy Priest Lake humidity. The thermostat reads one spot in one hallway. Treat the ranges here as a starting grid, then tune by a degree at a time until the house feels right at a setting you can hold steady.
Steady is the key word. Air conditioners and heat pumps do their best work, including removing humidity, in long even runs. Big manual swings, cranking it down when you get home and up when you leave, usually cost the comfort you were trying to buy. More savings ideas that go beyond settings are in our guide on lowering HVAC bills in La Vergne.