As far as I can tell, the concept here is somewhat different than in many other types of games where you can levy and then adjust a tax rate to get more money, at some other cost in citizen satifaction, etc.
Here, you are earning your money through various types of commerce and then spending it on entertainment (the happiness slider) Science research, etc. (The only two that you control directly and immediately - the rest need a bit more work).
The rest of your money goes on unit costs, is lost through corruption, and is paid out for maintenance on some buildings, etc. (So in a way you are paying "taxes" rather than collecting them).
In other words, you are not gaining your major source of income through imposing taxes, as you might in some other games. However, you CAN turn some of your cities' citizens into tax collectors who will "collect" one gold per turn. You will of course still need to feed them so the 1 gold will come at a production cost (probably more in shields, bread etc than the 1 gold is worth). So I don't think that making tax collectors is a major plank of most strategies. More likely just something to suit a specific situation. There's some info on p.177 of the manual.
