EDIT: Found it. If you really want to get technical (and I like to), seat backs DON'T have to be up for taxi, only take off and landing (but CO policy still supplants this):
Title 14: Aeronautics and Space
PART 121—OPERATING REQUIREMENTS: DOMESTIC, FLAG, AND SUPPLEMENTAL OPERATIONS
Subpart T—Flight Operations

§ 121.571 Briefing passengers before takeoff.

(A) The placement of seat backs in an upright position before takeoff and landing.[/QUOTE]

As per the Flight attendant safety checklist prior to push back from the gate seat back, tray tables, headrests,footrests and monitors stowed"