When decreasing and increasing taxes, the morale of your city will either increase when you decrease your taxes or decrease when you increase the taxes.
Taxes are 1 out of 2 ways of gaining platanium (money) and you use platanium for research and for updating Bell Tower and Gargoyle Perch.
Every percent of increase or decrease on taxes changes the moral of your city by 1%.
Morale is for how much population you have. 100% Morale is full population limit, 0% is no population in your city (deserted city). If it says 84% then you have 84% of your total population limit already.
When attempting to achieve maximum platinum amount for your city, the max can be gained with a 50% morale. So 50% tax without the building a Nightclub.
The tax rate will be found in the production tab in your Downtown. Picture below shows this well.
Also the Nightclub admits you to raise your taxes a certain amout with out a decrease on morale. A level 9 Night club permits 21% increase on taxes (from zero) with no morale loss.
Morale should be equal to tax rate for the most platinum, so with a lvl 9 night club, 60 or 61 % tax is the best.
If you want to spend a completion upgrade on your Nightclub, this raises the amount of "morale-free" tax to 25%.