I have a bright yellow ResqMe seatbelt cutter/window punch on a loop of stretch cord around the turn indicator lever. It is snug and isn't going anywhere, even in a rollover or major collision. In addition, there's a Leatherman in the glovebox, a folder clipped to my pocket (usually) and a little SAK on my keyring, so I think I'm covered.

Over 3+ decades in the business, I've seen a few seatbelt buckles that were jammed closed, and more, since they are toward the center of the car, that were inaccessible due to either wreckage or other things, typically the patient's body, being in the way.

One of the main reasons for a seatbelt cutter, IMO, is to be able to exit the car as quickly as possible if it's still in the roadway, before another vehicle comes barreling along and slams into the wreck.

BTW, if a car was in a significant wreck but is being repaired for some reason instead of junked, any seatbelts that were in use at that time are done and must be replaced, even if they look undamaged. So be sure to include that in your insurance claim.