See, this is why I haven't decided on whether to agree to cervical checks - not because I'm worried about infection, but because it seems like dilation and effacement alone mean nothing. Two of my good friends had opposite experiences. For one, the doctor said, "Any day now!", but she finally had a c-section three weeks later. For another, the doctor said, "See you next week!", but she had her baby by noon the next day. Same doctor! So there is just no telling.
That said, Pregnancy, Childbirth, and the Newborn claims active labor begins at 5cm. Not sure how they know that, but that's what it says.