I know that when I made my lastest unofficial 6th quest in its classic form....I did Dungeons and Overworld as I went. I had an overworld prepared, but since some of the dungeons go in a linear fashion, I had to do the dunegeon connections as the dungeons were created.
When you need the quest tested, you can only do so much through planning...so you want to do make a partial overworld than some dungeons and then another bit of the overworld.
That's just me though.
~Jay