Immigration enforcement is increasing and the focus has shifted from employees to employers.
If you haven't already, consider keeping your I-9s in a location separate from the employee personnel files. That will make it easier to pull them in the event you are faced with an audit.
Also consider having an I-9 audit conducted internally--either through your own HR staff or with the assistance of your counsel. It's much better for you to find the errors than to wait for USCIS to find them.