Tilt! Tilt! Tilt! Tilt!
Using head tilts together with a strong body gesture will strengthen your animation. Always consider 3D space when using a head tilt. Look at the drawings I've included with this post. The head is often tilted in a combination of directions. Using the same drawing and simply angling it up or down is not a true representation of expression. It's only action so a character doesn't freeze.
Take a look at the difference between the upper poses and lower poses in this drawing on the left. See how changing the tilt in 3D space makes a difference even though the expression has stayed the same. Sure the upper poses can work in many situations but look at how much more the lower set accents the look simply by giving the head a tilt in 3D space. Granted my character designs lend themselves to work in 3D space but often I find people making stylized cartoons take the lazy route of saying "well my cartoon style is flat". To me this is just a lack of trying. Even the stylized cartoons of UPA and Disney used head tilts. Experiment with your characters and see how you can make them expressive while still fitting into the design of the film. Sometimes a head tilt can become part of the style of your film.