It is all about speed and territory.
Get as much territory as early as possible and win as fast as possible.
That goes for all victory condistions. Trying to conquer 65% of the map should always be one of your goals in gotms.
If you go for a conquest or domination game, this of course makes choises easy. Now there is only one goal, conquer the world as fast as you can, jason only gives you the extra assignment not to raze any cities, but to keep them all and try to fill the gaps between your newly conquered pieces of land.
If you go for 100k, it also goes pretty much hand in hand. 100k victory needs massive amounts of cities with temples and libraries in them.
When going for a 20k victory, this causes a conflict. You would like to invest all your resources in that one city, but now you also have to maintain a war to conquer an empire. Early in the game is best to conquer an empire, but this is also the most important time for your 20k city. You must find the right balance to do both and get the best score.
When you go for a UN or Space victory, it works better again. Research is now the goal. While you would like to build libraries, units can also greatly help your research. They conquer land that can be turned into specialist farmland. Of course you will do both the libraries and the specialist farms. Because of the jason system, you have reason to give a little more priority over units and build the libraries a bit later than you otherwise would.