For me personally, I want 3-4 tiles to be the ideal distance without it being forced on the player. There should be good, genuine underlying reasons to spread cities out, not a restriction that reduces options.
This is why I like flexible resource bonuses on buildings and I'm glad to see they added this mechanic. It naturally rewards placing cities in ideal spots, because a single city can efficiently boost many resources with one building. One city with a granary can boost X resource tiles at half the production and maintenance cost of two cities with two granaries. It rewards strategic city placement instead of placing roadblocks in the player's path.
With 3-tile city spacing the advantage of having lots of tiny, undeveloped cities isn't inherently changed, the player can still ICS but simply over more land area.