depends what your shoulder and back workout look like. i do 5 exercises for my back so i just throw shrugs with shoulders. also i do upright rows for shoulders and that involves the traps when you squeeze up top so i just do shrugs right after upright rows.
I personally do my traps with both shoulders and back. For me, it's like doing calves and abs, I can do them more than once, I don't always do traps twice but do it often!!