This occurs at the opening of the game. I usually build my first city right on the spot the settler starts with. I can't look up the shields etc in the city map because I am one turn away from being able to do that.
So, if there is a chance by building on another square I might get more shields and thus lessen the time for building settlers, how on earth do I know how to compare the square my settler starts out on with another square since the city in not yet built.
In Civ1 you don't get the free shield when you build on unshielded grassland squares unlike in Civ2. If you look, at the start the Aztecs have a shielded grassland square, as do the Romans, Zulus, etc. The French and Americans build on unshielded grassland which doesn't give them a production.
By the way, ever noticed how crap the Romans, Americans, English (of course), and Chinese do on the world map? They often don't expand beyond one city, as they build too many units for their city to have any production. Giving the free shield on normal grassland seems to have helped fix this problem - it was an aid for the AI it seems.