Whenever the question appears to be salvagable.
(Otherwise, just downvote and/or close vote it.)
If you feel uneasy with the edit, why not simply leave a comment for the OP (and others) why you changed the question's wording? To provide an example, I chose one of the questions subject to Ewan's post, did such an edit by myself and added a related comment. As you can see, my comment got an affirmative response from the OP. When you explain people why you changed their question, then in most cases it should not be a problem, quite the opposite.
What bothers me more that in most cases, already downvoted questions most times keep their downvotes, regardless of any edit, and our well-known downvote trolls do not leave a message what else bothers them about the post, or if they have noticed the change, or why the change don't suit them. The inept close-votes fortunately age away automatically.