I found this to be a very interesting question. I’ve always seen it represented on the right side, either in movies or with crucifixes. But last year when I started reading for my parish, I was in the sacristy preparing and noticed for the first time a stained glass window and a crucifix both with the piercing on the left. I asked the priest, who pointed out that the crucifix also showed our Lord with his eyes still opened – so that definitely put accuracy out of the question.
This article helped to explain a little more:
Taken from (http://catholicsay.com/which-side-of-jesus-body-was-pierced).