Kuningas said:
When I'm going for fast science. I irrigate everything, food is power. (not settler/worker factories).
On the other hand if I go for early military I mine everything.
If i go for science, i first conquer half the world with an optimised core, then irrigate everything and support as many scientists as possible. (think of 300 cities supporting 600 scientists)
For settler worker factories, irrigate what is needed to get that 5 food per turn. And mine what is needed for the production. this is another chapter though, find info on factories in other posts and articles.
Early in the game, in despotism, irrigating grassland is a waste. So then mine it, and irrigate plains.
Later on, you can freely mine or irrigate what you want and it is the total amounts that matter. while growing cities from size 6-12, i prefer to irrigate so much that i get around 5 food surplus per city if they have to grow on their own. More importantly though, i choose the tiles they use targeted on production. I do not want 9 production, i want 10, i am willing to give up one food for that. When it has grown, maybe it would get 11 production and i swich tile usage to get 1 more food again and have it stay at 10 production.
If you have food boni however, it is nice to use cities that are not in your immeadiate core, but at mediocre distance and corruption (50% or so) to be worker factories. These workers can then be joined to other cities. Do not keep core cities below size 6 to be such factories, that wastes too much opportunity.
Usually it is a combination of both. Some cities will have a good production as well as a good growth(bonus grasslands, cows), these can grow by themselves.
Others can have a good production but will have a hard time growing (all plains or many hills), there i will join some workers from the factories.
When all your cities are maxed out, minimize food to target for 0 surplus. When your cities start to produce polution, give them 1 or 2 surplus so they keep their food boxes full and don't starve when they lose a tile to polution.
This all is only for your core of course. If you have captured corrupt lands, irrigate everything there.