Is it more economical to run a central AC unit with the fan always On or should you have the fan running in Auto?

Not necessarily. I would caution you to watch the Relative Humidity in the Home. Depending on EACH system, the amount of moisture that lays in your drain pans is then put back into the conditioned space when you continuously circulate the fan. If you are in a tropical environment, this is not the most efficient way to operate your system. The home may actually "feel" warmer when the air conditioning comes back on. This effect is caused by the changing of moisture in your home. Moist air is lighter than dry air, so when you start the dehumidification the moisture rises. The removal of moisture is the biggest cost associated with the conditioning of the air. You be the judge, as you will notice whether you are cooler and saving in your electrical consumption. Yes. It is more economical to leave your fan setting "on" continuously. This way it circulates the air throughout the house which makes it cooler and decreases the time that the AC unit acutually has to run. It does not really wear out the fan motor so don't worry about that either. If you will leave it on, you will notice that you are able to turn up your thermostat and your energy bill should decrease slightly.