the twelve principles are nice. But don’t overdo the exaggeration squash and stretch part, unless you want to become a rubberface disney disciple.
American animation has one traditional rule: drawing animation (and now 3d) must be funny and for kids. I am more into japan animation, where the squash and stretch is limited to healthy amounts. It depends on what you want to achieve.
I must agree with HauntHouse, allthough, there are times where even Disney’s productions broke that squash and stretch rule… And these were masterpiece films…