only up and down.
Dont remember where, but I read something (not sure if it was a study) that talked about the stress put in the shoulder joint when rolling the shoulders.
I am sorry to say that I dont remember why though.
My thought is that, because the trapezius is such a strong muscle, you need to lift a heavy amount of weight in order to overload it.
When you start to roll the shoulders and add a small external rotation component, you are putting the some of the rotator cuff muscles under a significant amount of tension.