While a 5 pronged chair is not required, it is highly recommended for stability purposes. Again, the recommendation comes from the Human Factor Design Factors Book 2nd Edition. Don’t have the book, get a copy on Amazon here http://www.amazon.com/Factors-Design-Handbook-Wesley-Woodson/dp/0070717680
Check the very bottom of OSHA webpage below for the reference to the 5 prong chair.
Cal-OSHA governs ergonomics in Section 5110 http://www.dir.ca.gov/Title8/5110.html
Standards for chairs can be found here: http://www.bifma.org/standards/standards.html