Do pilots using an electronic flight bag need to carry paper charts?
A EFB is a legal replacement for paper charts and you are not required to carry backup if you are operating as part 91 single engine piston. Advisory Circular 120-76B is in reference to part 91F (Large and Turbine-Powered Multiengine Airplanes). 

AC 91-78 is aimed at Part 91 operators, VFR or IFR and states that EFBs can be used in all phases of flight in lieu of paper.

A backup data source is suggested, but is not required. Note that this backup can be another electronic device.

See iPad Legal Briefing for Pilots.