You can also use a vertical line on the ball and work from the mark it leaves on the face. I find for people not keen to hit balls from a solid surface this works great and can yield some good results and allow me to track contact relative to the centre too.
That is the way I had my J33's adjusted and I thought it was much more accurate. As you say, if the lie angle is correct you will hit the center of the face.
I suppose either way would work as long as you do each club in the set. Visually, however, I think having the ball leave a line on the clubface is a better indicator of where on the face you are hitting the ball. Tape on the sole of the club can sometimes be hard to read where the mark is when you hit off a lie board.