Postmodernism, in its most explicit form, not only questions the existence of truth, but proclaims its non-existence. Let's set aside the paradox that one cannot make a proposition that there are no truths and have that proposition be true. Instead, let's look at this from another, more practical, perspective.
If there is no truth, then there are no lies. Certainly, if we remain in the relam of philosophy and not in the realm of practicality, a case can be made that there are no lies. However, consider what the response is when you are on the receiving end of a lie. Little else brings out your sense of truth, your sense of right and wrong than being the recipient of a lie.
Lies are so damaging to relationships. Lies rupture trust, throw into question how a relationship will progress, and perhaps forever change the relationship even with forgiveness because once there has been a lie in the relationship, there can never not have been a lie in the relationship. You know that there is the capacity for a lie to be executed in that relationship.
In a bizarre twist of irony, lies may provide some of the best evidence for the existence of truth.