I did it in one of my first games against the leader while we had a NAP thinking anonymous spells were ok.
And I did get called out on it - gee, who could be casting murdering winter? Niefelheim, hmmm...
But it was more of a joke then a blow up.
After thinking about it, I decided not to do it anymore, mainly because the gains are not generally worth the loss of trust if you are caught.
But I can see the argument either way. When I did it, I considered it like the espionage that goes on between allies in the real world. If you get caught, it could lead to a blow up, otherwise, it is just an accepted fact of life.