I'm not really sure. I had no pain when I did them, and I think it's because my body could adjust itself to what it found comfortable.
I just let my arms and shoulders go where they needed to, and I had no pain. Does that make any sense?
One thing I did notice is that my arms would twist ever so slightly while benching. They would turn so that the DB's were closer to, but not completely, sideways to my body. (Kinda like you would hold them for fly's, but not completely parallel.) They naturally moved that way.
Hope this helps.
Joker