No, boss souls are never worth it to consume unless you are speedrunning. They don't grant that many souls (12,000 or 16,000 may seem like a lot of souls but it really doesnt take that long to acquire that through farming/normal pve).
How many souls is a fading soul worth?
50 souls
What do you do with fading souls?
The Fading Soul is a consumable soul item in Dark Souls III. The fading soul of a corpse. Use to gain a smattering of souls. Let the Fire Keeper transform this sovereignless soul into a source of strength, for to be Unkindled is to be a vessel for souls.
How many souls are in the soul of deserted corpse?