I think it's important to learn HTML/CSS. A good understanding of user interface will complement and affect the way you program. You should have a good grasp of how a website should "flow."

On the other hand, graphic designs is a different story. Developing a good artistic sense will take time and practice. If you plan on developing entire websites on your own, why not spend a little time to graphics design to make things look prettier? However, you don't plan on spending any time on this, then find a good designer :)