I thought both panels of glass on the Z1 are DragonTrail glass? Seems like it's tough stuff.
I've seen a couple of scratch tests on youtube, and they both went very well. One used a scalpel as the final tool though, and IIRC that started to scratch it up.
I'd always leave the ASF film on the phone, and if it starts to get scratched up, I'd just pull it off and replace it with a new one. Keeps the glass in perfect condition, plus if the glass did shatter after a drop, you would be able to keep using the phone without risking cutting your fingers up. Makes sense to me...