Knowing your way around CSS is absolutely necessary if you want to have a half-decent looking website. In a lot of ways, having good CSS fundamentals is the great equalizer - if you know your stuff, your sites will look good, regardless of whether you're a freelancer or a bigger agency. Think about it this way: You could be one of the best SEO agencies in the world, but if the sites you're ranking don't look good, no one's going to end up using them.