I storyboard, but only because I like having a concrete idea down before I proceed, and I can still make little tweaks and changes along the way. Then again, I do more comics and things, so that's just how my mindset handles things.
Azure has the right idea. You need to have some sort of storyboard, so as to avoid any unexpected deviation from the pace and tone of your animation. If you have a set plan, you a greater sense of control over your project and its better to play around within your own storyboard and decide on what you'll put in before-hand instead of playing around/experimenting within your actual animation. This should guarantee a higher quality animation.
I'm not saying animations that aren't storyboarded won't be higher quality, but it is definitely more beneficial to storyboard, whether mentally, on paper, or in a sketch.
