IMO you shouldnt need one. The factory screen protector is made out of a soft film and is only there so the screen doesnt get scratched during assembly, QC, and transport. You can get away with keeping it on the screen because it is so soft. If you were to get a normal screen protector for a tablet or something etc I would think that it may be thick enough to dull down your inputs to the touch screen. A cell phone or tablet etc benefits from a screen protector because they constantly have a cover on and off the screen or can be layed down/slid around on surfaces that can scratch them. The only way you are going to scratch a screen on a navigation unit is if you stick your finger in mud and then rub it all over the unit.
Also you mentioned the last thing you want is your image quality to decrease. If you put a screen protector on it surely will as even the anti-glare ones dilute the picture and still have some glare. Good luck with whatever you decide but if it was me I would leave it alone.