One of the many unpublicized aspects which set the iPad high above the competition is the glass which is used over the display. The high quality of the glass might not be that apparent until you use a tablet from other manufacturers. Now you will see how durable and thick the glass is compared to thinner glass and even plastic on quite a few competing devices!
A good rule of thumb when using a tablet with lesser screen quality is: definitely use a screen protector! But, with the iPad, a screen protector is not near as important (some could argue it's not needed at all). My take on the situation is, if you are the only person using it, and you are careful and use it the way it should be, then there's no real need to cover the screen with a protector. But if you have children or other users, then it just might be a little added insurance to use a protector. If you have a salesperson strongly pushing a protector (especially high cost ones), feel free to turn this down. It's not a life-or-death mandatory situation.
((I've added this post since I've used touch devices for so many years and have experienced all types of screens and screen needs. Most of the time I've used some type of protection, but most devices did not utilize the same quality of glass as the iPad does.))