The problem with doing this is that you might get too much damage on the add so it reaches 1% before it's outside so it will wipe the raid. It's probably not that hard to pull off, I've just not been touching them below 25% because we have other people who are trigger happy in my group. We've also been tight on the DPS on the boss so even if I would get soul shards by doing this I'd essentially waste a couple of seconds and possibly cause another add to spawn. I'll try it out when we clear it this week though.