In a pex household, would it reduce corosion to bond metal plumbing fixtures to my water heater's anode?
Metal water pipe isn't grounded to minimize corrosion, it's grounded to protect people in case something goes horribly wrong and the pipe gets energized.  With PEX, the pipe is an insulator, so grounding is both effectively impossible and not beneficial.

If you were to ground any water pipe or fixtures, you would tie them to the house ground, not the water heater anode.