I believe involving HR should be a last resort, if at all, unless one is very clear about the role of HR in a corporation. HR is not there for the protection of the individual employee. Put another way, HR is not on your side. It is there to protect the corporation and minimize its risk exposure. This does not mean that people who work in HR are bad, simply that HR and employees often have competing objectives and priorities. In my experience, many employees are not cognizant of these facts and end up escalating issues to HR only to find an unflattering paper trail left in the wake.
In terms of documentation, I think the wisest thing do is to save damning or abusive emails (in hard copy form), memos, and voicemails for a rainy day. They could be useful when negotiating severance or any number of other situations. Generally speaking, the only person or entity with your best interest in mind in the employment scenario is you. Be careful when allowing any other person or department to "advocate" on your behalf.
EDIT: Big caveat--in the situation where you, as a supervisor, need to document and support the reasons for firing someone, it is a good idea to involve HR early. The fact that HR can be most useful when firing someone should be enough to make someone think twice about going to HR for anything except benefits issues, etc.
Edited by The Monkey - 7/25/10 at 10:03am
