You use whatever bio media works for you, there's no hard and fast rule. Don't be taken in by the marketing hype of certain bio medias which claim to offer this, that and the other. They tend to be expensive and, more often than not, a much cheaper alternative does just as good a job.
Some people use just one type, others, myself included, mix it up a bit. I have sponges, plastic bio balls and ceramic rings in my bio section. And don't forget, although your bio section is your primary area for BB, they are also on every underwater surface too. BB will colonise every area of your set up, they really aren't fussy.