When is air bridge needed for CPW?

Is there a rule if the air bridge should be used at the discontinuities of CPW? As for CPW shunt stub, some one used the air bridge and some one did not use the air bridge. How do I determine if the air bridge should be used?

My view is the best way to address this problem question is with a 3D electromagnetic simulator and a model that is specific to your application.

You can then see how the structure behaves in your specific case.