One day you guys will come into the modern age of metric.

As for risk, you can look at it this way:

If you're going to make the trip to the store, you might as well have a go. If the SIM is wrecked you still have to go to the store. Err on the side of caution - i.e. cut bigger to start.

They will definitely not trim your SIM. They will replace it. Sometimes here it will cost you $20-30. In Australia at least you just go and get a new SIM and the old one is cancelled. The new one is then activated on your account. Takes minutes here.