Keeping your fan on after your temperature has been reached will cause humidity in your home to rise. Your coil is wet and after reaching desired temperature keeping your fan on will cause humidity to rise. In cooling "auto" is recommended.
Greg
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Fan On vs Auto by: Shawn
The fan switch on your thermostat is just for indoor blower motor. If it on ON, it means your indoor motor will run nonstop until something break.
The ON position does not have anything to do with your condenser fan motor.