Turning the thermostat down is fine if you are just considering reducing heat lost through the insulation while the hot water is just sitting in the tank. Consider what happens though when you want to take a bath. Because you will not be adding any cold water, you will require the entire contents of your hotwater tank to take your bath. If you live alone, there is no problem. If your partner enjoys cold baths, again, no problem, but if 2 or more people want a hot bath, they will either have to wait for the tank to reheat, or you will have to heat your water to a higher temperature so that you can get two or more baths out of it when mixing with cold water.