You pre-exert Traps more with shoulders than you do with back imho but saying that I usually train them with back like most people do.
If I think about it sensibly though it would be best to train them with shoulders as shoulders is an easier routine than back due to the muscle size/excertion. Traps are a fairly big muscle on their own so I personally do a good 9 sets of shrugs in a pyramid fashion. Its all down to personal preference though but if you think of the size of your back and train it all properly then it takes ages: Deadlifts, rowing, pulldowns/pullups etc etc Done properly I would say back is the second most involved workout after legs.