Personally when I draw my snubby from my pocket, my thumb automatically goes on the hammer. Thus no snag. I like the option of having a hammer to cock if I have a need or desire to do so.
Thats my preference and may or may not work for someone else.
It (putting your thumb over the hammer), may not work for you if you have been jumped and beaten to the ground, stomped as you are fighting to get your pocket revolver out.