If the DK is taking threat when he should not be (Saurfang etc) then that is his fault and not yours. A tank is never doing considerable enough DPS to warrant me using my salv on him over a real DPS during a fight, and all i'd be doing is enabling bad tanking habits.

Actually I do salv tanks once, and only if I am only pally. Thats for Festergut when we are swapping and they have 8+ stacks.

If you are in a guild run and having problems with a DK threat being an issue then you need to talk that out with them. If its a pug, then just stop running with DK tanks.