Lepios

I would say that in the situation you outlined:

Dirge
Mystic
SK
Inquisitor
Coercer
Assassin/Brigand/Swashy

I would personnally put it on the Assassin/Brigand/Swashy just because it will result in a decent increase in their dps. I would probably never put it on the coercer just becasue there are better ppl to put it on. If you have the proper gear to support it (proccing items like hammer of the pureblood, hoop of the chosen, band of the chosen) and you need extra healing due to high damage on the tank, then i would say yourself. Finnally If you are in a heavy melee dps group and the dirge in the group has tombs calm up, which they should then it may be benefical to place ancestry on them to bump up tombs calm across the group.

Ancestry is better much best on ppl with a large amount of proccing items. Since most items have a 1.8 ppm rate with ancestry its pretty much double in reality even thought it doesnt say 3.6 ppm.

Diotrum

My normal raid setup:

Support/DPS group nearly always goes to the high DPS Scout. I always pick the scout over casters because they use EH poisons and are geared for proc DPS.

OT always gets it when OT'ing, otherwise high Scout in group.

Myself or Temp (if grouped) on fights with heavy AE dmg due to 3 items that proc group heals and claymore.