When do you prick your pie crust?

You prick a pie crust before baking to allow steam to escape from the pie crust. Otherwise the crust will bubble up.

The reason you only dock part is because in order for the crust to bake properly it needs to shrink from the sides of the pie plate. If you puncture the entire area, this will not allow it to shrink properly and will cause the sides to bake too quickly.