This topic is also addressed in the Appraiser Active MAYHEM post.
The September issue of LiveValuation Magazine has an article written by yours truly on the subject of workfile maintenance and retention. Maintaining Your Workfile was written to remind appraisers your first line of defense in the event of a suit or complaint is your workfile.
UPDATE: Over on the Appraiser Law Blog, there is a new post with information about the statute of limitations for suing appraisers. It's worth reading, if only for this quote:
Because this is so frequently misunderstood by appraisers, let me say this first: the relevant time period for suing an appraiser or AMC about an allegedly bad appraisal has nothing to do with USPAP's minimum 5-year record keeping requirement. (Don't throw those workfiles away!)