That all depends on where you're seeing the hyphens. Also, it's possible the "hyphens" property is not the right thing to use depending on how the hyphens are getting added. The most helpful thing you can do is link to an example on your blog where you are currently seeing hyphens so someone can help you take a closer look at a specific example. It's possible WordPress has some PHP programming in place to prevent orphans which overrides CSS like you are trying to add.
To answer your question more generally, the XX needs to be a CSS selector that matches whatever text you're targeting. Also, keep in mind that the rules I mentioned earlier only apply when soft wrapping happens.
So, in the Twenty Twelve theme, to target widget titles, you would use this as the selector:
.widget-area .widget h3