For me, it depends on the situation. Sometimes when I lift the camera to my eye I think "what am I taking a picture of?" If I can't answer the question, then I don't shoot. Part of it is not wasting film, not for financial reasons, but that I'd rather save my shots for better opportunities (especially if I'm out with limited rolls). On the other hand, if and when I get into the "zone" I just shoot and don't even think about it. I like those situations best since I tend to come up with some real keepers, but they don't happen often.