Oh, I will dispute the "need" (if by "need" you imply it is too hard to do without) to guard healers even there.
I have done it even on hardmode without guarding healers. Besides if you were to ever do it in 16-man you would have less tanks than healers, and could not guard them all, and would have to device a way to keep aggro off unguarded healers anyway. Although I did only 8-man in HM, I still did not even bother guarding healers.
It is only before the first tank switch that you run a high risk of loosing aggro to healers. So all you have to do is to have the first tank to begin by using both his single taunt and aoe taunt to build threat, then few seconds later have the second tank to taunt it and use both his single taunt and aoe taunt to build up threat. If that second tank is Assassin/Shadow he can also stand in the outer circle and use Force Shroud/Resilience to make himself immune to the damage he receives for 5 seconds, so there is not even any pressure for healers to switch targets all the time. Then after 5 seconds the first tank will taunt it back to himself, and hold it until the first scream when they need to tank switch for real. After having done all this the boss has been taunted so many times, that with the way taunts multiply your threat, the tanks will no longer loose aggro to healers, and will continue to just tank normally.
The last 20% is a bit tricky too, but it can be handled too.
Does it benefit to guard healers in this fight? Yes it does. It is one of those rare fights where normal aggro mechanics are not in place so it actually does benefit to guard healers.
Is it necessary to guard healers there? No, not really. You can do it easily enough even without it.
Taunts don't build threat independent of other threat already being on the target. Throwing both of your taunts right away just gives an overlapping 6 seconds of the boss attacking you - the amount of threat it will give you is dependent entirely on the highest amount of threat already on that target.
When you're taunting to start a fight, you're not actually gaining threat. You're just forcing the boss to attack you for 6 seconds.
It's not impossible to do without guarding the healers, I didn't mean that. But it makes it a lot easier, and there's no reason whatsoever to guard the DPS there.