I would bet that using the heater wouldn't hurt your gas mileage by any amount you could measure. Yes, it would use some engine heat, but since that's thermostatically controlled, it just means the cooling fan has to work less. Other accessories using power would be the blower fan and the rear window defogger. The latter probably draws significant current, so I would turn it off before it times out if it's no longer needed. However, all of the above loads are probably quite insignificant compared to the way you drive.