Hi,
You can use both depending on your needs. All LCCS components are written in AS3 and you can always use itemrenderers or CSS.
We have used both for building applications using LCCS. Even in LCCS code, you will find itemrenderers used in Roster.
Similarly, if you want to define styles, you can use CSS files and use it in your project.
Hope this clarifies things
Regards
Hironmay Basu