I've personally completed about 120 hard-elite shrouds and epic devil assaults across twenty characters since dopants have been available in the game, and I've had zero drop for me outside of 20th lists.
Clearly in this thread, someone has used three so the drop rates have a high variability, but from what I've heard, most people are in my boat.
Dopants are quite valuable and I would suggest you only use them if you have utmost certainty that you will not use a particular greensteel item again.
Even then, I advocate keeping any and all greensteel you have made (even if it's a khopesh and you're on a monk life, for example) simply because I think the sunk cost of making them and their future possible utility outweighs any possible use you may have for a couple of larges and a signet stone.
The most certain example of what I'd decon is probably an item you've actually crafted the wrong properties for (screwed up dual shards, etc).