I use slightly different rules for upgrading, but then I play huge maps/marathon speed.
Often (in fact most of the time), I've around 20-30 cities at the time of Gunpowder. Often my continent by then, is for all intents and purposes mine, or at least peaceful by then. The main threat is going to come from overseas.
I still believe in having one mega military city, with all the settled generals, mil acad etc. This city I use almost solely for building up a very large "attacking army", all starting with lots of promotions. I do not build city garrison troops here. What I personally do, is as soon as I discover Gunpowder, every coastal city drops whatever it was doing, and builds at least 3 City Garrison Muskets. Once these are done, they'll probably (in most cases) never build a troop again until the modern era. I update them all slowly , and at suspected points where the ai will inevitably eventually attack given first priority(I've learned from experience, that sometimes your power rating just doesn't matter).
Then the "expert" troops from the mega military city, are ready in one place to launch an immediate, and probably decisive counterattack. It's just probably "playing safe", but to have all these distant cities building first 3 muskets, then 3 rifles, then 3 infantry, would mean (with often low
output), that they were building nothing else, and to have other more developed cities build troops for them, would compromise the bigger cities ability to quickly build the nice shiny new buildings.
I'd rather they built banks and universities, while I upgrade the already built defenders.
But then it
always depends upon game situation......
for 1 
. Once I switch away from HR, though, I delete those units to save on maintenance.
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