Centerpunches only work on side windows (unless they are laminated). They are useless for the windscreen (=laminated).

As far carring a knife. It's a delicant balance between scaring people and having a good knife with you. I don't fancy getting fined or arrested for carring a knife. But lifehammers and resqme's are quite effective.

Mine experians with cutting seatbelts is cut at a angle. Not at 90 degrees. Much easier and smoother.