So, when you buy a case for the iPhone 3G, many come with 1 or 2 screen protectors bundled in. SwitchEasy, for example, gives you some when you buy Colors. Are those good enough to protect your screen, especially from drops (to the most practical extent possible, of course). Or are they only protection for scratches and dust, and in order to get drop protection for your screen do you need to buy something like the Best Skin Ever or Zagg InvisibleShield -- and if you do have to buy these, how can you be sure your case will fit over them? Thanks.