I need to play a few more games to get a feel for all this. But yeah, there are some cases where a new city should be settled upon reaching Monarchy (like in one of my toku games indeed). But it has to be a city that has a great mid-term commerce potential, otherwise it will just ruin your Optics date.
On one hand, the cities I settle after reaching Optics usually don't start producing units until 1000 AD, and on the other hand I just can't settle them before Optics. Maybe a later, stronger attack is the solution? If you're going for Cannons+Rifles, the newly settled cities usually have enough time to pay off.
It also shouldn't hurt too badly to get out a few settlers before Optics and settle the cities on the Optics turn. This would also require very good worker planning, but nothing impossible.
The thing I'm becoming really sceptical about, though, is Alpha. Building research and no infra puts you in such a production hole later on when what you need the most is hammers...
Anyways, like I said, gotta play a few more games