Although it is not necessary under Church law to confess venial sins, it is wise and beneficial. Consider the advice of these four saints:
Saint Augustine: "In failing to confess, Lord, I would only hide You from myself, not myself from You."
Saint John Climacus: "Confession is like a bridle that keeps the soul which reflects on it from committing sin, but anything left unconfessed we continue to do without fear as if in the dark."
Saint Faustina: "A soul does not benefit from the sacrament of confession if it is not humble. Pride keeps it in darkness. The soul neither knows how, nor is it willing, to probe with precision the depths of its own misery. It puts on a mask and avoids everything that might bring it recovery."
Saint Josemaria Escriva: "At the time of your examination beware of the devil that ties your tongue."
Lastly, compare the benefits that the soul would receive by concealing vs. confessing one's sins. Pope John Paul II went to confession every single day (and I don't imagine he was giving his confessor a litany of mortal sins in there). Therefore, the confession of our smaller imperfections can do us a great deal of good.