when to use Over Under Grip

for a lil while i had been using a over under grip while doing BB shrugs. i found it easier to hold the bar as i added more weight. the thing is i was talkin to a guy that is a personal trainer and he said that i shouldnt be using this while doing bb shrugs because it hits more trap than the other, he even went on t say that one trap was a lil bigger than the other (i was wearing a T-shirt)?after thinking about it i thought it made sense but wasnt aware that using that type of grip have such a negative effect while doin shrugs. i know its best to use this grip while doin deads but is it really so bad while doin bb shrugs?

Without a doubt the position of the hands builds the traps differently.