Less trips to the gas station also means a little saving. Plus I read somewhere it avoids rust forming in the tank since there will be less area for condensation to form in case di masyado ang gamit ng car.

All in all, I'd say fill it up if you can afford it, but never over-fill. Pag automatic na, stop na dahil mag-eexpand pa naman yung gas.

Kung nagfi-fill up naman ako, I don't add more fuel until 1/4 or 1/6 na yung gauge para balanced.