it does not matter, either back or shoulders, i do them with delts, if you do deadlifts it is probably better on back day.
i used to do them on back day, but when i started doing upright rows, i put shrugs after them on delt day, uprights hit your traps harder than deads(in my opinion)
i get better results with delts, but up 2 you, try both, dont take someones word for it......do it your self.