I am also using a screen protector from day one that needed to be cut on the edges, due to the N9 screen curvature. Trust me it feels great to know my precious N9 is protected.
The original rubber casing was a bit loose, and I was removing it all the time to clean it. Those tiny bits entering between the housing and the casing will scratch the housing - guaranteed.
Since then I purchased a tighter rubber casing, I wrapped the phone in one layer of plastic foil (kind you use for wrapping your sandwitches), placed the phone back in the rubber casing and trimmed the excess. 1 minute job.
Naturally, I cut a small round hole for the camera, speaker and usb cover, but left everything else protected by the plastic. In this way I hope I can add a couple of extra months to the lifetime of the buttons and shiny finish at the camera.

Love your N9