For me the ideal case is three-state. When run interactively with no flags, print a dry run result and prompt the user to confirm the action; and choose a default for non-interactive invocations. In both cases, accept either a --dry-run or a --yes flag that indicates the choice to be made.

This should always be included in any application that has a clear plan-then-execute flow, and it's definitely nice to have in other cases as well.