If you are playing a long campaign, it is likely that you will want to have not too large a population for your biggest cities. While having a larger population does give more tax income, the increase in tax income after a population of >35,000 is minimal and even for the 7905 - 35000 bracket, the increase in tax income may not justify the need/cost of a bigger garrison to maintain public order.
Therefore it is reasonable to maintain a population of about 24,000 (just enough to get the highest governance buildings). A population of 24,000 means 80% squalor and -8% growth. With this in mind, your decision whether or not to build a health building should be partially based on achieving an 8% growth to counter the -8% from squalor and to maintain a balance of 0% growth so that population remains fairly static at 24,000.
This in turn depends very much on the base growth (base farming level) of each region.
Cities like Mediolanium/Tylis/Sinope/Thessalonica/Sardis/Halicarnasus/Memphis have a base growth of 5.0 and therefore need another 3.0% to achieve 8.0%.
IMHO, having the following will make up the other 3.0% nicely:
Low Tax to maintain better public order +30% (0.5% growth)
L1, L2 or L3 Trader (for more trade income) (0.5% growth)
L1 Farm (for more farming income) (0.5% growth)
L4 Health building (Aqueduct) for better public order +15% (1.5% growth)
In the case of cities like Carthage/Patavium, these have a base growth of 7.0 and therefore you may need to settle for a slightly higher figure, say perhaps 8.5% growth with:
Low Tax to maintain better public order +30% (0.5% growth)
L1, L2 or L3 Trader (for more trade income) (0.5% growth)
L1 Farm (for more farming income) (0.5% growth)
If you again build all health buildings here, growth will increase to 10.0% and squalor will balance up growth only at a population of 30,000. The basic increase in squalor will be exactly countered by the additional health bonus to public order but maintaining an effective garrison at a higher population will be that much tougher and the added wealth from taxes by having this larger population may not compensate garrison costs.
You could raise taxes to Very High to curb growth but sometimes it is not possible to do this while still maintaining a public order above riot level.
In short, IMHO, regions with very high base growth may not need health buildings but for most other regions, having health buildings should help get that desired 8% growth to counter squalor.
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