There's a bacteria called Legionella that thrives between 60 and 120 degrees Fahrenheit. To prevent it from growing in the hot water supply, most water heaters shoot for 140 and then achieve cooler temperatures by mixing in cold water at the faucet.

Your plan could still work, but I think that the water heater is its best target, and that much of the savings goes away once you factor in that constraint.

I think that there are many opportunities to improve by thinking along those lines though. For instance, I always thought it was weird that the grocery store I worked at ran the refrigerator compressors even when it was cold outside.