Stealth triggers any time that you could use a disguise roll, but your disguise roll is worse than your stealth roll. If you have no stealth skill, your stealth roll is a rock bottom 1, always. In other words, if your Liberals can't sneak, they don't bother trying.
NOTE: In Version 4.06.3, I have noticed that, though untrained, he still does sneak occasionally (Noticed while in the Gentleman's club - He disguised many many times, but also snuck once.). Thus, I believe that Stealth is still used, but that Disguise if almost always higher when it is untrained.
However, all is not lost. If you want to develop a little stealth skill, try stripping naked and streaking in public. The embarrassment of it all might cause your people to start going into stealth mode and develop some stealth skill. Or maybe it's the -100 disguise you get from being nude. One of those two.
Note that while disguise is generally a more powerful skill, having both very high is very good, as you'll only be spotted when both rolls fail.