With our guild we run naxx 10man and 25man. My brother is one of the raid leaders, and he assigns tanks based on who is best suited for the situation and who has tanked what, leading me to have done most roles in 10man and 25man.
When tanking bosses or large mobs, I use seal of vengeance, and never have to worry about mana regen. When I am in a situation where my damage intake is less, I switch to seal of wisdom, and never have to worry about mana regen.
When trying to stay second on the threat list for a given fight, the issue is more that I need to hold back then that I need to worry about mana. I find it quite easy to overaggro on a lot of fights.
Of course, I've always had at least one shadow priest present, and on 25mans, there's usually four or more paladins there, so I have all the blessings there are.
(And remember, seal of wisdom gives a percentage of maximum mana, so the more mana you have, the more it gives back, so stacking kings, gift of the wild, arcane intellect and a guru's elixer helps too.)
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I'm about to drop the hammer...
...and dispense some indiscriminate justice!
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