The traps act as a synergist muscle during any press, dumbbell raise, or rowing movement. You can target the upper parts of the traps better with things like shrugs, but I don't think shrugs are entirely necessary if you're doing everything else.
If you are going to be doing shrugs, make sure you go through a full range of motion and hold the contraction at the top of the lift for a moment. Also, after each set, flex the traps for a good 20 seconds or so.
I have a cousin who plays football for Marshall and his traps are freaking insane. Most of that came from heavy compound lifting, especially the deadlift, not the shrug.