From mostly everything that I've researched and everything I've been told Seal of Insight should be used while tanking at all times. I've recently joined a new guild and went on my first raid, as well as my first time, in MV 10M. I did well aside from cleaning some things up but I was approached (very respectfully) by a member of the raid group (plays Hunter/Priest). They said that I should be using Seal of Truth when single target tanking at all times due to increased DPS as well as increased threat, and was adamant that I wouldn't be able to hold aggro with their hard hitting dps. I explained my case but they said healing from insight was negligible (I was 9.4% healing on overall data.) I don't feel my threat is a problem at all, not with that group and not with harder hitting DPS. I am glyphed with Glyph of the Battle Healer as well.
Is he right? Should I maybe start out with Seal of Truth then switch to Seal of Insight once I've established threat? Or stay with Seal of Insight? Everything I was sure of is now gone due to that comment and I just want to make sure that I am playing my role correctly.
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Fist of Justice
Glyph of Divine Protection
Glyph of the Battle Healer
Glyph of the Alabaster Sheild
*Adding in the message in it's entirety. For those that were curious.