To be fair, I don't run much Nightmare content due to my work schedule. Hard to find a group on Sunday evenings that wants to run Nightmare on my server. I am sure they are out there, but I am still relatively newish to progression raiding and I am content with the challenge that Hardmode gives me. Just to clarify, taunt fluffing is AOE taunt then single target taunt some time in the first three to six GCDs?
Sort of. Usually you single-target taunt first to get it off CD as soon as possible. The maximum threat opener for a shadow tank is as follows:
Pull > Sprint + Project > Slow Time > Force Breach + Single Taunt > Double Strike > Project > Force Potency + Telekinetic Throw > AoE Taunt + …
That is taunt fluffing. You can do that on most bosses, but unfortunately some do have mechanical restrictions which prevent this (see Thrasher). In those cases, you can sometimes swap off between tanks to increase threat, and you should do that if the opener is causing you issues.