The B-screw controls the body angle of the derailleur. It pulls the pulleys away from the sprockets, so you don't rub against them. If you set it in the largest rear cog (as you should), when you're adequately clear, you don't have rubbing.
Its a somewhat insensitive adjustment once you clear the cogs, but the closer you are to the loosest screw value which works (i.e. the closest pulley to cog), the better your shifting will be. If you set it too close, note that a bicycle moves, so you need to allow some tolerance due to vibrations and what not. Set it too far and your shifting won't be as crisp as it should be: "Since a derailer shift is caused by forcing the chain to run at an angle, the greater the angle, the sooner it will shift. The closer the jockey pulley is to the cluster, the sharper the angle will be for a given amount of sideways motion of the derailer. Thus, the looser the angle adjuster screw is, the better the shifting will be." (from Sheldon)