Personally, I work most of the time as a
Stage Manager, and I will always prefer an operator to wait, or ask me instead of just second guessing my calls. For example, in the last show I did, during one of the performances we had to call a doctor twice, due to first a crew member and then a cast member having fainted. In those circumstances, you don't always have the time to explain what is going on to the operators, because you are busy trying to check that everything is ok, with the people who have fainted. At the same time you are still there to remain on the book, and thus it is the job of people around you to look after the 'casualty'.
Basically, I would say that if you think you should be standing-by for a cue, then ask the SM, however in emergency circumstances, things do have to change during a show, and that is why you shouldn't take a cue unless its called. Also the key thing to remember is that a designer may have changed the placement of a cue, and there is no obligation to tell you this..