At this time, I really dislike how terraforming is done. The problem is, I find you get the best terraforming results if you hold off until you get all the terraforming techs. This means holding back until you get to the third age of the game, something that might not happen in a lot of games.
There several reasons for holding back:
First off, there is a terraforming improvement that can terraform all level 1 terraformable tiles. Because you can terraform less advanced tiles using more advanced terraforming improvements, you have to watch out to prevent yourself from doing just that. If you do so, then that terraforming improvement will have been wasted once you gain the ability to terraform all the level 1 terraformable tiles. You would have more tiles if you just waited. As such, I hold back until I can terraform all level 1 terraformable tiles, as terraforming them first will eliminate any possibility of making such mistakes with level 1 tiles once I start terraforming more advanced tiles. By holding back, I feel that I maximize the number of tiles that I can get on my worlds.
The second reason is, it is easier to plan things once you have them all terraforming improvements. You can maximize adjacency bonuses once you are able to see what you can get. I start with selecting the most advanced terraforming improvement to show me what I could possible terraform, and then work my way down. At first glance I might see many tiles in an area that can be terraformed, but it might not prove true if the less advanced terraforming can't fill in the rest.
I miss the simpler days of GalCiv 2 where you didn't have to worry about such details. No worry about adjacency bonuses and no risk of wasting terraforming improvements.