No reason not to mark if you've the spare keybinds to do so quickly. Nothing says people can't throw dots up on targets two or three.

And if it's a new healer or new tank, I've no issues with folks sleeping mobs. It lowers outgoing dps, but also lowers incoming dps, and if it means the tank doesn't have to dodge a cone AoE and screw up somebody's positionals, the dps loss is (somewhat) mitigated.