this is very interesting topic. I thought about it a lot. Let's say that I'll give a homeless person 20$. After becoming happy a little bit he maybe will run to buy some food or maybe (unfortunately) drugs. But there's a chance that by doing so he might get hit by a car or a lot of other things might happen to him due to event triggered off by my action. So I can't know for sure what decision is better (actually I believe that in some really rare cases I can (for example if I have results of a reliable scientific research)). Also I remembered Jacque Fresco when he said that he saved a child. And people said what a good act he did. But he said that if this child will become Adolf Hitler then he would say that "Oh, what have I done?!".
I know about Eisenhower principle which states that the first things we should do are those that are important and urgent. After that we should do those that are important but aren't urgent. Then those that aren't important but urgent. Well, it seems to me that it might be a good principle to make choices (but I rare follow it).