The guideline is: if your method does something that the programmer should think twice about or shouldn't use without proper knowledge (e.g. didn't read the docs), use !. An incomplete case of usages:

  - There is a safer alternative (e.g. mutating vs. non-mutating or skipped validation)
  - It should only be called once in a process (e.g. Padrino.start!)
  - It is non-reversible (many statemachine libraries use action! as the way to invoke state transitions, which might not be reversible)

This doesn't mean that every method needs to be suffixed by ! if it does something destructive. `delete` in the context of an ORM is standard, so it doesn't have a bang. The whole point of `pop` is to manipulate the receiver: no point in warning about it. IO is always destructive, so ! doesn't make sense either.