Post shading is best done after the last flat coat is applied. And RemcoGrob is correct, a sealer coat makes them disappear somewhat. So far I am doing all post shading. The models are not handled much, and dusting is very gently done, so there isn’t any worry about it coming off. It does not come off as easy as that anyway and can be easy touched up if need be.
You mentioned control and that is exactly why I like post shading. Plus it is part of the weather process for the makings as well. Pre-shading will not show through a very opaque set of markings. The pics below show a heavy application on the 109G and a more subtle, less worn look on the 190D-9.
