Simon I have to disagree with the very high tax rate.

I have done the OPPOSITE. A lot of my provinces where on high tax rates and I found it much cheaper to lower the taxes and disband the unneeded militia.

This is town depending of course, if the tax difference between say normal and very high is over 1000 then of course it might be different.

But remember each militia unit costs something like 100 per turn.
Changing an average sized cities (10,000) tax rate from normal to high only gives to an increase of 100-200, whereas disbanding those 3-4 extra militia needed just for the extra tax saves you 400!

I only raise more troops as a last resort - as a rule I try to keep everything on normal tax or higher (depending of course), but if the city needs to go on low taxes and there is nothing else to build (temples, government building, sewer, public path, whatever, depending on the problems) then I will resort to more militia.

Anyway, one of the biggest costs that was making me go broke was the AI building troops for me. It would fill my cities up with expensive Auxulia skirmishers, so I went around and turned off the AI recruitment in all my managed cities.
Now I have about 40 family members so I dont even need AI goveners. :)