How do you determine when a dough should be docked.
It varies, and for somewhat different reasons. For example, many skins intended to be used to make thin-crusted cracker-style pizzas are frequently docked. Also, skins that are to be pre-baked/par-baked before completing are usually docked. Typically, you will find that the skins for the above applications are formed by commercial sheeter/rollers, or by rolling pins in a home environment.
Some pizza operators will also dock skins intended to be used to make other styles of pizzas. For example, Papa John's docks all of its doughs for its American style pizzas. Others will often dock their doughs when they want or have to work their dough when cold. As with the other examples, that is to reduce or minimize large bubbles forming in the finished crusts.