You do get the bonus grassland effect after size 6. I did an experiment on this once. If you build on a +1 shield tile (ANY tile, not just bonus grassland. That is, hills, plains...), you get +1 shield later on.). A metro will get +4 shields in the capital square when it's on a bonus tile, hill, plains compared to +3 for normal grassland. It's one of those wierd things.