I follow fairly simple process here:
1. Make sure I am aware when Patches or Expansions are going to arrive
2. Read around the other gold blogs to determine what changes are going to be made in Inscription. I would recommend this blog, most blogs you see on my blog roll and in particular AFKCrafting
3. If the are only going to be a small number of changes then I don't do anything. The gold making opportunities are unlikely to eat into my supply of raw materials.
4. If there are likely to be major changes then I hold an extra week's supply (so from 2 weeks to 3 weeks). The third week is held in herb form, not milled into pigments or inks at this stage - no point in closing options off. You never know, the alchemists may come over the hill buying up the herbs are huge prices.
5. If the demand is such that I go through my entire supply of raw materials to craft glyphs as happened in cataclysm then I find the prices all rise more than enough to make a supernormal profit from the now inflated prices of herbs.
6. And as an emergency backup I have herbalism.
for this and other ideas follow this link to Croda's Inscription Gold Guide