Please clarify: when can a derivative join a pending I-485?

Suppose I file EB2 I-485 now and my wife wish to join me later (file her I-485) as my dependent, for example, in 09/2007.

At that time, suppose my I-485 is still pending, but my priority date becomes NOT current, can my wife file I-485?

Some say it is ok so long as before my I-485 approval. Other say my PD must be current.

Who is correct? Both?

Please share your expertise or experience. Thanks.