As a designer I've had a few requests for very tight tolerances on FR4 PCBs, usually Class 3 Military or Aerospace applications. Although it is possible to achieve greater than the standard +/- 0.010" from fabrication it is poor design practice to use an FR4 based structure in products requiring extremely tight tolerances due to material properties. Metal structures are usually the best solution, whether it's a bonded heatsink/carrier or a PCB core that extends beyond the FR4 edges.