I working them on both shoulder and back day as they are a lagging muscle group for me ... I no longer squat or deadlift (flippin knees / getting old) .. but trap dev is mission critical ....
Back day = strug
Shoulder day = trap machine / + whatever the exercise is called lifting 35 pound plate from arms straight out in front then raised direct overhead (great trap exercise) ...
I must be lazy, I have never been able to be bothered to load up a bar or even take the time to do shrugs at the end of a workout. Plus I feel like they get plenty of work with deadlift variations and bent over rows.