Pick and employee, any employee. The most important document in that employee's personnel file - and in EVERY employee's personnel file - is one you probably don't give much thought to until the employee is not performing, not responding, or you are wondering if you can dismiss the employee. It is the Acknowledgement page from your employee handbook.
Generally placed in the back of the handbook, the Acknowledgement page states that the employee has received a copy of the employer's policies and has had the opportunity to read them. Normally, there are both a contract disclaimer and an at-will acknowledgement as part of that page. If you update your employee handbook, you need to have new acknowledgement pages signed by all employees and kept on file as well.
If you are not having your employees sign this page and then keeping it on file, your business is at risk - at risk of taking an employment action based upon a policy that you cannot prove the employee had notice of. This can lead to an award of unemployment compensation and irtcan be used against you in a discrimonation claim. All of this can avoided if you simply make a regular practice of ensuring that ALL employees have signed their employee handbook's acknowledgement page and returned it so it can be placed in their personnel file.
For those employers that have moved away from paper copies of their handbooks and instead put them online, you need some other form of acknowledgement documentation so you can show that the employee is on notice of the policies.