Are you doing deads weekly too? (I think my dead and traps are related)
For myself, and since a shrug (specifically) is a short ROM move, I could do them 2x per or after a dead session or after back work or even squats. I seem to get a good W/O using higher reps too ie: 15-20 say.
Even if you are hitting them once per week, if that is more focus than you have been giving them, they should respond with more growth than no direct training at all. As far as the length of them goes, that might be more determined by genetics, as some guys traps seem to extend farther out. EIther way they can be made larger and give a more goloidy gorilla look IMO.
I have also done, at the end of a set of low cable lat pulls or other lat work, finished the set with say 5 rearward lat shrugs, to further beat up my upper back. YMMV.
IMO same thing goes with forearm and or grip work 2-3 times per is probably plenty.
And of course, quality of a few sets of an exercise, will always trump (using Wendler's lingo) doing 3-5 different moves, with less intensity and or determination.
