Placing shadows on III, I've almost never seen X in the mountains. If you're working in the "Golden Hours", contrast ranges will certainly be reduced in all but brightly lit clouds (one hopes for!), that's why those hours are golden for photography. I rarely need to contract more than one zone (N-1) at those times. You may however, have to make a choice when 7 or 8 zones
are present in your composition (
example), based on your comfort with contracting your film's development. Are the skies most important or the resolution of shadows?
I've never used a GND with B&W films (or, at all for that matter). Those decisions are made in the darkroom, where burning in skies is simply the law. Most of my time under the enlarger is taken up with skies. AA had to do it in nearly every general landscape. Filters stronger than deep yellow will almost certainly wipe out your hard fought for mid tones, particularly in low sun conditions, resulting in harsh, gritty prints (insert beaten dead horse here). Frequently, blue sky will be the same zone as the clouds, and once again a yellow filter will go a ways to reducing burning times.