How accurate should sun\moon ephemeris be for calculating eclipses?
I suspect this is fundamentally a question of error propagation.  My personal favorite reference is "Data Reduction and Error Analysis" by Philip Bevington.  Basically,  once you have the formulas in place,  such as  

eclipse_time = f(sun_ephem, moon_ephem) 

You take all the partial derivatives and "stuff" them into the error propagator formulas along with the known or estimated standard deviations (error/uncertainty) of your input variables.  The uncertainty in the output  just pops right out.