Simon,
"Opening a song can't be undone in any program that I'm aware of."
That is not entirely true. "Any program" would open a "second, third, etc." (song/document) without affecting original. That's the reality. If anything destructive is being done to the project - warning is usually displayed. That's how most programs that I am aware of function.

The best practice in my view, since BIAB can't host 2 or more songs at the same time (Unlike most programs), is to have a proper Undo that reverts last action.