Also as a side bar question, how many of you use the governor during an ICS? Do you set the governor to "emphasize gold" or "emphasize science"? Or simply leave at default or manually adjust the workforce?
Before turn 40, it doesn't matter. When building a settler, food=production, and there aren't many improved tiles to work. The governor doesn't have enough available options to make a mistake.
I usually hit the Classical era with Construction around turn 45-50, depending on the ruins I got. When I start researching Construction, I go through all my cities manually to assess what is going on. If one of them is, for some reason, not building a settler, I'll try to arrange the tiles so it has enough food to grow right when I finish Construction.
Any workers at this point have solely been on luxury and road detail...haven't had enough spare actions to build any regular tile improvements. I do try to build roads through riverside hill tiles when feasible...farming those is the next priority.
When Colosseums are going up, I emphasize Production. The key to REX+ICS is using your limited happiness on working city tiles instead of the less productive regular tiles. The Production-emphasis Governor does a pretty good job of picking the right tiles when the city is small.
Once I am researching Currency and pushing towards Banking, I do another reassess. One or two of my cities will become gold-specialized. In a recent game, I had one early city that happened to have 4 gold/silver mines in its third ring. Early on, I focused my settling around that city to claim that land. Once the Mint was available, I built one and locked those tiles for that city. Market and Bank were next. After that, it was a size-4 settler pump.
A windfall like that made gold-speccing a city early extremely valuable, but it's still worthwhile to do that with tradeposts. Just remember about the overlap...you don't need to put those tile improvements right next to the spec'd city. Build them and lock them and let them run.
The third time I do a full city revaluation is when I'm running out of expansion room and need to start thinking about vertical growth. Before this, I've been focusing on river-farms with the workers. This speeds settler production and gets me ready for going vertical. At this point I generally put everything on Food focus to get my science skyrocketing.