So half of this thread is now grammar discussion and inane paragraphs...
But if you're the religious type, a third thing that'll cause it to go off is commonly known as an Act of God. Otherwise, they should be perfectly fine. Unless your display shelf is right over the fireplace? Still probably pretty safe, but not something worth risking.
As to varying types of rounds, I know this kills your gun-buying argument for the girlfriend, but there are tons of websites that sell single rounds specifically for collectors. Someone had a thread a day ago about that cause he was looking for some rather rare calibers.