While the ideal situation in most fights is for the tank to have the adds as well, the DPS can tank them in most situations, so be wary of anything that prevents you from doing so. One of the biggest example is any boss with hard hitting conal attacks. Use ranged to snag aggro if you can, but if it comes down to letting them tank or hitting them with the boss's attack, let them tank. There are also some bosses where the tank may not want too many adds on them: a prime example is the first boss of Pharos Sirius. The mobs there do a fast, almost unavoidable attack at half health that inflicts a debuff. Getting hit by 3 stacks of the debuff causes Bad Things, so it's preferred that each add be tanked by a different person.
