Everyone is instructed to do this. The General Instruction of the Roman Missal states,
At the ambo, the priest opens the book and, with hands joined, says, Dominus vobiscum (The Lord be with you), and the people respond, Et cum spiritu tuo (And also with you). Then he says, Lectio sancti Evangelii (A reading from the holy Gospel), making the sign of the cross with his thumb on the book and on his forehead, mouth, and breast, which everyone else does as well. (GIRM 134)
Former This Rock contributor Fr. Mateo offered the following explanation for this ritual: "For the word which Christ brought and which is set down in this book we are willing to stand up with a mind that is open; we are ready to confess it with our mouth; and above all we are determined to safeguard it faithfully in our hearts."