I will offer the counterpoint from personal experience.
Crown Grill may hold back some reservations for "suite" / "casino" passengers. (I stated 'may' because I don't have personal experience with that situation.
I do have personal experience with immediately upon boarding going straight to CG - sorry, booked all week. You can stop by during the cruise at dinner hours and see if we have seating for a cancellation.
I can book you into Sabatini's, but only for tonight ... we took that ... and continued to call the dine line at various times during the cruise - nada, sorry.
So, it is unfair to posters to think that the dine line is magical.