In general it is wise to not use the slider any more than you must.
nbarclay has a thread on the power of the slider that breaks down the details.
In Regent and Monarch games you have the first two born content. After that all the level above it have only one.
I play a lot at Emperor and the second pop is going to be unhappy, unless you take some action. At first that will mean an MP. Soon you will need to have a lux hooked up or use the slider.
Once you get a temple, you get some help, but it may still require 2 MP's, a lux and the slider, depending on the size of the city.
You may replace the slider with entertainers, if only one or two cities are impacted by unhappy pop. In short, the slider is probably a must in all of my games and will seldom be set to zero, once used. I may have to crank it up to 30 or more at times.
About the only way to not use the slider is to play at Warlord or Chieftain or get all the luxs hooked up or traded for.