I should note that i play at a rather low difficulty Warlord.
Ok here is how it works, and so far i have had two games (both Greek Civ) with no civil wars.
Once i have picked Slavery, i check the dissent page, if a city is working towards less then loyal i use Slavery to hurry production. Sometimes what ever is next in the que, sometimes i pick something that increases healthiness or happiness.
This often results in return to loyalty or at least increases the number of turns that it takes to reach stable.
I'm not sure if this is how its supposed to work, but that is the effect.
The only effects Slavery should have on dissent is from the civic itself (high dissent), and dissent from the unhappiness generated by whipping. I'm guessing that dissent is lower in the city because you're killing most of your unhappy citizens. Check the dissent advisor - which column is showing lower dissent when you whip?
Overall dissent is pretty low on Warlord difficulty, wouldn't surprise me that it's possible to have games without civil wars there with the current settings.
Also i noticed that when i have a Vassal and they have a CW that creates a new Civ that new Civ declares war on me and then is also at war with my other Vassals if any.
Seems to make for some very short lifespans for these 'rebel' civs.
I'm currently working on ways to make breakaway civilizations less 'diplomatically isolated'. Sometimes the AI is going to be stupid no matter what though.