My Rule Of Thumb is: Build Metros if you have to, otherwise just build more cities....It's cheaper & you don't have to deal with pollution.I've often researched Sanitation in upper-level space-games and then watered everything for more scientists. I haven't figured out any way to determine if this actually pays off though. Nor determined how often it would pay off instead of not paying off.
(Unfortunately), an exception to this Rule applies to most of my own (current) games.....HOF Huge Histographic.......The limit is 512 cities on the map (including the AIs') and you WILL need to have many Metros if you want to Maximize Game Points!
