Depends on how much you saw and how bad you think it was. If the offensive players ends up on top of the defender on the floor then you probably need a whistle, right or wrong. If you think the L or C could be no-calling a defender leaving early or something like that leave it alone. But if you have two players on the floor then you probably have something. In certain situations I think a total guess is better than nothing at all. It's all a feel for the situation and the temperature of the game.