I think it just depends on the program...I know Emory would like for applicants to have alittle experience but that doesn't mean you won't be offered an interview without any. I personally have some but the other people I interviewed with didn't.
And from what I could gather from my South interview, only a few out of 14 had experience....So with all that being said, if you have it I think its a plus. If not, rely on your academics, references and personal statement.