I'm not convinced that continually topping up a few litres at a time is making any real difference.
I am convinced!!! It really makes a difference!
But logically thinking unfortunately a negative difference...I don't know how it is with you all here, but normally I drive a few hundred meters, sometimes even a few kilometres extra to get to a petrol station. This causes burning some extra fuel. The more often one would do that the more (unnecessary) fuel one would burn.
I refuel our Jazz roughly every 650 - 800 km. (400 - 500 mi) and Roman suggests to refuel at least 3 to 4 times more often... How many extra (unnecessary) km./mi would one make over the years?
Next to that: 1 liter of petrol weighs about 750 grams. If you never empty your tank beyond, say, about 30 liters left, you are always driving with a minimum of 22 kg. petrol "ballast" around. And as we all know: extra weight means extra consumption...