When in doubt, label it. But also when not in doubt. And when you're just trying to be extra cautious. Labeling is like sprinkling fairy dust on your design - it's magic, it's wonder, it's a surefire way to attract the attention of your users. Unless, of course, you're trying to attract the attention of your users' therapists.
So, when to label? Label whenever you feel like it. Label when you don't feel like it. Label because your cat told you to. Label because you're secretly a fan of 80s new wave and you're trying to make your website look like a rejected Devo album cover.
But seriously, labeling techniques are essential for: