I answered in the other threads, I have a screen protector on my IPP, but there are MANY protectors out there and each one covers a different personal preference. The glass seems to offer more protection for breakage, the matte to reduce reflections, etc.
The screen protector I installed on my IPP is very simply the most like the original screen that I have found, it does help with the fingerprints/smudges doesn't completely remove them but it helps a lot. It is so clear and sharp, I can't even tell I have one on my screen! No rainbow effect, no screen discoloration, doesn't darken my screen at all. The Pencil works exactly like it does on bare screen with just the tiniest bit of extra tooth, but no dragging at all.
This is what I have on mine and I really recommend for folks who want to keep their screen as original as possible but just like to have a screen protector on their screens.
https://www.amazon.com/gp/product/B0159Q72CI/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1
One thing I'd add is if you purchase this one is too give it a wipe after install with a slightly moistened microfiber cloth, it will help to remove any factory "stuff" that may be on it, when I first put mine on I thought ugh I don't know about this...until I wiped it down, now I'm very satisfied.