I just filled up last night - my 10th straight 19+ around town tank; mine gets 24-26 on the road. If all else is working correctly, I'd look elsewhere for a gas mileage issue. If it were running that rich (so bad gas mileage were the symptom), either the O2's would pick it up and the ecu would correct it, or you'd get a check engine light indicating a problem it couldn't correct.
tmoss - my sense is that any sort of radiant heat issue might show up during hot or cold starts - but once significant air is flowing over the business end of the sensor, I don't think things would be greatly affected.