My upper back/traps has always been strong. I have been using shrugs in my routine for 7 years. When I first performed shrugs, I was using a 185lb barbell and 90lb dumbbells. Currently, I work out with up to 405lbs on the barbell for 6 reps, and 140lb dumbbells (heaviest that the gym supplies) for 12. I like to hold at the top position for a two-count and return to the bottom position. Heavy shrugs have helped my deadlifts and squats tremendously.
I never perform shrugs on the smith machine.