Your thermostat is the heart of your HVAC system. It can control both your AC and your heat, and in that way, it also controls the amount of money you end up spending on your heating and cooling bills every month. Just a single degree difference can make a major difference in cost. Sometimes, it might even be the case that a faulty sensor in your thermostat ends up costing you money because of not reading the correct temperature.
If you have a very old thermostat, you definitely should consider upgrading to a digital thermostat. These are generally much more accurate than an old-fashioned thermostat, and they can have many additional features that can help you control your temperature much better. You could, for example, schedule times for the temperature to be set for a lower or higher setting, depending on the season, for when you are out of the house for work.
Lowering the temperature a few degrees at night will help you sleep better as well.