I’ve written more extensively on the subject here: Unquoted attribute values in HTML and CSS.
I’ve also created a tool to help you answer your question: http://mothereff.in/unquoted-attributes
You can usually omit the quotes as long as the attribute value is alphanumeric (however, there are some exceptions — see the linked article for all the details). Anyhow, I find it to be good practice to add the quotes anyway in case you need them, i.e. a[href^=http://] won’t work, but a[href^="http://"] will.
The article I mentioned links to the appropriate chapters in the CSS spec.