I always take a picture of the dashboard showing mileage and fuel gauge upon rental and upon return. I haven't had to fight over tenths of a gallon read by these automated systems yet, and I suspect if the information available to the consumer (the gauge) is visually consistent it would not be in their best interest to refuse a refund and potentially attract the attention of regulators/AGs/lawyers. I also make sure I have the return receipt before I leave the premises. Of course if they make an adjustment after the fact you'll have to deal with the agency remotely, and they can't say "you should have handled this at return."

Also, I've used enough "free" gas from filling up 10-20 miles from the return (especially when a Manhattan rental and there are no convenient nearby options) such that if the gauge drops below 8/8 shortly after pulling out of the lot, I am happy to let that slide once in a while and return above the Full mark...