The top-level B+W filters are some of the best in the world and I wouldn't think twice about using them even on my most expensive optics. The same can be said about Heliopan and numerous other top brands (all of which I use).
However I most often use all my filters on their own and only rarely stack them unless light levels become extreme. A lot of the time a PL filter is enough, and tend to use Grad ND filters as much as full ND filters.

Yeap, no arguing there. They are definitely one of the best. I am against stacking filters as well but this is exactly what Vari-ND/Fader-ND are all about. I would stick to B+W 106 but there is a major drawback; you have to constantly put it on and off because focusing becomes impossible. Vari/Fader NDs are very convenient in that regard, just turn and you can see clearly again.