Yeah I know how you feel, a degree in this stufff don't mean one knows what they're doing, I got one year of uni to go and I have to say, if I relied on what we've only learnt at uni I'd really suck at this. I bet you can still get a job over someone that has the experience but no qualification, hence the reason why I decided to go back to university.
To answer your question, if it's a whole CSS layout yes you would need it, CSS layout can be so confusing if you've always designed in tables, i.e slice it up in
PS and lay it out with tables, my latest website, I wanted to do the whole XHTML/CSS layout but it had too many bug, I just scrapped it.
Even if it's not a whole CSS layout (tableless) you still need CSS to make things look neat/uniformed etc.