Your shoulders should be tight through the whole dead. Your arms do not just 'hang'. (or should not)
I have gotten 'cocky' and shrugged heavier deads for the heck of it. I would not recommend it though. Some times you get fired up when you can rip up something easier then you thought
High rep deads really target the traps better then anything. The heavier the better. "heavy" is a relative thing, but you can really punish yourself on some 15 rep even 20 rep deads. Brutal and you will be sore in places you did not even know existed.
I with regular deads, I dont really see the need for shrugs. I get all the stim from deads and rows. Arguably my traps might be one of my best developed parts....and I would have to attribute that do the deads....
Thread: How do you get big traps?