i think it depends... and i guess the 2 of u wil noe best.
in the end u haf to decide on how u see her intention of lying -
whether its to protect u, protect herself, or protect the
relation.
and also whether this is a perennial problem, on and off, or just
once in a while.
if u think shes hiding something i think the best is to let her
know. and try to talk about it, rather than having bad feelings
inside.