There is no problem entering on your Advance Parole if you are approved on your trip.
If the I-485 application is approved after the applicant's departure from the U.S. In that circumstance, it is typical to grant deferred inspection, so that the person can go to the local INS office and obtain the permanent residency stamp in the passport.
Deferred Inspection at INS Office
If the local INS office confirms that the I-485 was approved, parole should be terminated and the applicant admitted for permanent residence. If an approved / pending application is not located at the particular INS office conducting deferred inspection, the particular INS District office or Service Center which has the application should be contacted before a final determination is made.