I used to think in terms of "best use" per square, mapping out the ideal end city. I've come to another point of view on this.
Now, I look for flexibility. I like to see city tiles that allow me to switch the city production around. The key is to have tiles with a range of food production - 0,1,2,and 3 - so I can combine them efficiently when I stop city growth. Producing food to create an idle citizen because of health/happiness seems a waste. Why not get higher trade/hammer with the city size I have?
So I look to make the best use of the citizens, in a flex way. Most of my cities have a normal mode and a production mode, for when a new tech means there's a buidling I want. Libraries or markets for commerce cities, forges for production cities, and courthouses for everyone. Once the improvement is built, its back to the citiy's function. I even like work to give the GP city the option to switch on some hammers.
For instance:
City has a farmed plains (3 food) and mined grassland hill (1 food, 3 hammers).
Those two citizenss can be swapped out with 2 coastal/lighthouse tiles 2food, 2 commerce each) with no change in food supply. I get to choose whether the city gets trade or production.
Seeing this, I end up using workshops on the plains squares whenever there is a companion square to provide the food. I can always replace that workshop with a farm when I want the growth. But for a while, that +hammer square is a nice toggle. I can use it for production, or I can drag the guy off and turn it into a specialist. These are nice tiles to have. Sort of two way options.
Just a different slant, I guess. I love to micro manage.