I worked a lot of MVAs in EMS. We never needed to cut a seatbelt because the buckle was stuck. We had to cut them because the buckle was hard to reach or because the belt was snarled in debris.

+1 to the comment about hooked cutters, if there's any slack in the material they will usually hang up. Happened to me with the Benchmade Rescue Hook.

The fix for this problem is so low cost, low hassle, and effective that I would keep an appropriate tool on hand.