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

The main function of the mounting tabs, besides of course providing ground connection, is to hold the connector in place and prevent it from damaging the tracks on the PCB during connection of the cable.

It's not clear to me. http://www.hardwarebook.info/Universal_Serial_Bus_%28USB%29#Shielding says to connect the shield to ground only at the host, which makes sense from a ground loop perspective, but the discussion at http://forum.allaboutcircuits.com/showthread.php?t=58811 shows standards that point to direct connections, connections through a ferrite bead, and connections through a capacitor!

The big cahuna, the USB2.0 Standard at http://www.usb.org/developers/docs/, says

6.8 USB Grounding The shield must be terminated to the connector plug for completed assemblies. The shield and chassis are bonded together. The user selected grounding scheme for USB devices, and cables must be consistent with accepted industry practices and regulatory agency standards for safety and EMI/ESD/RFI.