It really depends on the purpose of your drawings. If you need to show your stack / plumbing for the purpose of building regulations then I would suggest you continue to use the 2D symbols. It is easier to create the various components, label and dimension them.
You can achieve the same using 3D models, but it is much harder to create / resize 3D elements than 2D. However if you do use the 3D route then you are still able to add text dimensions in the 2D section / plan views.
I understand why you may want to show the external WC stack within your 3D view, but using 3D for the 'other plumbing' is a bit redundant because this is normally under the floor / behind the walls and would not be visible anyway in 3D, unless you want to create section views to demonstrate.
