Can't override matplotlibrc ?

It seems that I can't completely override my choice of backend.

E.g., I have set TkAgg as my default back end. Yet if I modify
examples/ to include matplotlib.use('GTKAgg') or
matplotlib.use('GTK') it will not run. A window appears, but it contains a
static, non-changing graph. To run, I have to modify matplotlibrc.
Same behavior the other way: with GTKAgg selected in matplotlibrc, I can't
run Todd's, even with TkAgg selected in the script. Am I doing
something stupid?