In my opinion, all materials should be shown at actual material thicknesses. The drawings need to be correct - and reflect accurate structural and material thicknesses. If trades have a particular way of rounding up/down/etc, they are entitled to do so. As what Andrew Dwight stated, a block layer can add 100mm, but it isn't actually correct. From an architectural point-of-view, I need the walls and materials to be accurate on my drawings. The trades can simply adjust these drawings as per the methodology of the area (country, state, etc).
This is why this thread diverged slightly, in discussing the possibility of showing the structural as well as finished thicknesses of walls.