But then you really got to weigh your options... should I crank out that extra maceman/treb or build a two spies...
By the time I've got Engineering I normally have enough cities to not need to build units in all of them, so personally I'd switch a commerce city from building a Market or something to Spies. Or if I had a tundra city I'd build a spy from the overflow from whipping a Courthouse.
If I did have to choose then it would mainly depend on how many cities with castles there are