USB micro B receptacle has 6 mounting tabs. How many do I actually need to ground?
I think normally you don't ground the USB shield on the device side. Instead, you should connect your shield to the chassis/other shielding components, and connect your PCB ground to the USB ground. Between the USB ground and shield you can add a 1Mohm resistor in parallel with a 4.7nF ceramic capacitor.

References:

Cyprus Semiconductor: Common USB Development Mistakes

Atmel: USB Hardware Design Considerations

edit:

did a little more digging, and for higher speed connections it seems like you do tie the shield to ground? I'm not entirely positive about this.

How To connect USB connector shield