As long as it hasn't achieved orbit, all the pieces will end up on an elliptical track with the lowest point inside the atmosphere. Some stuff might get flung way up, but it would still need another maneuver (requiring working engines) near the highest point to circularize the track to prevent it from coming back down.
There's not much ambiguity for when to destruct. It's just: has it gone far enough off course that it could free-fall outside the predetermined borders where it's acceptable to crash? Then it's time to blow it up so it doesn't fly any farther.