I keep my PRS 650 screen clean with a small microfiber cloth similar to the one provided with the reader. (That is just one reason I want to design my own cover with a pocket, I want a place to keep a cloth, notes or receipts, but that's another show.) As far as sensitivity...Yesterday I was outside reading during lunch. A fly happened to land on the screen and I watched as the pages turned forward then back, individual words were highlighted, the little error circle lit up a few times. I would say that the IR sensors are pretty sensitive and may loose some specificity if detrius is allowed to accumulate.
By the way, I lost my appetite and went inside and cleaned my screen with something other than my little microfiber cloth.