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 :)
Having a good design "sense" is important - you should at least be good enough to tell excellent from good from bad. This goes doubly if you're in business for yourself, since you'll likely have someone else doing your design for you on a contract basis, it's good to know they're good at it.
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 :)