Many lcd screens, including those on some dSLR cameras already have a "protective screen." Basically the part that you see on the outside and can touch is a hard cover that is non-permanently glued on.
Some Nikon models like D3/D700/D300 have tempered glass over the display so chances are you won't nick it easily.
The screen on my camera costs like <15 bucks shipped from Canon, and can be removed with a suction cup and a blowdryer if it is scratched too much.
A "nice" glass one from Giotto's Aegis or something cost more than double that, and introduces two more reflective layers of glass. Of course then they go on touting antireflective coatings on the screen protector for your screen protector...
What I'm saying is to find out by contacting Nikon if your D5100's screen protector is equally removable and low cost to replace or if it's made of glass.