However, requesting a signature (or just an initial) on non-contractual type documents, especially screen-shots or data-schemas, is very beneficial. When you ask a client to 'review this screen mock-up' they will generally take a cursory glance, perhaps make a comment or two and then move on to something else. Asking them to then initial the print-out of the screen mock-up always makes them take a second or third look, ask someone else, or at least spend a few more minutes working out whether it's correct.
Training clients to continually review work carefully leads to better quality work.
Figure: A signature can be very valuable but sometimes hard to obtain