Hi,
The point of having the ashes in the form of a cross on our foreheads is to witness to ourselves and to others to the significance of God’s love for us as He demonstrated on Good Friday. To wipe them off right away is to defeat the whole reason for receiving the ashes in the first place. We ought to wash our faces BEFORE we receive the ashes and wear them the whole day. It’s an opportunity to witness to our faith.
But if we must wash or the ashes just wear off, it certainly is not a sin.
Fr. Vincent Serpa, O.P.