I know this post is a few months old, but I thought I'd add my 2c.
My E71 leather case has caused many fine horizontal scratches on the screen from continuously removing and inserting the phone. The scratches are from dust accumulating on the screen during the night while its on charge (and for some reason this doesn't all wipe off easily) and from when I leave the phone resting on top of the case at home (if it was constantly in the case, I'd have to keep pulling it out to check for messages etc., not very practical).
I tried a crystal case. Even though I wiped around the inside of this with a glasses cloth before putting it on, there was something stuck on the inside of the front case. Result: nice small scratch below the light sensor.
I don't use any case at all now. Sure the phone gets hairline scratches from being in my pocket, but no worse than what it got from the leather case.
I just keep it in a pocket on its own now, seems to be the best solution, and when its moving around in your pocket it wipes the fingerprints off nicely :-)