Screen protector is personal choice. Also depends on your situation/use of the car- if you have dogs in the car, kids, people that wear obnoxious rings, bracelets, charm bracelets, etc. that tend to smack the screen when they play with it. The screens are really tough as is, but people have broken and scratched it. The most common cause of chipping the screen is from tossing a backpack/bag/purse to the passenger seat and some hard piece on the straps or from said item nicks the screen. One bad case I read was a kid jumping into the car from the drivers side and swung his feet over to the passenger side while wearing some sort of sport shoes (soccer cleats?) and basically swiped the screen with nice trail marks.
In any case, it's about a $1200 to replace the screen vs a $20 screen protector. I didn't put one on at first, but after a few stupid close calls, I bought one. I went with a pricier version from Evannex only because it's the only review I found at the time that doesn't give you a rainbow affect when wearing sunglasses. Spiggen(sp?) is the popular brand, cheaper, and has a foolproof template to install it. Watch youtube installer vids to get an idea. I also went with clear vs the matte since I prefer to retain the sharpness in the display (I have 20% tint all around except front, so glare isn't an issue).