Anger can be good, it's a powerful emotional reaction to something that feels wrong and can lead to constructive changes if handled correctly.
As you said, it's not good to unleash anger when it's unfocused - when you haven't thought it through well enough to actually know what you're angry about. If you just relieve the built-up energy by throwing a fit, you won't accomplish much more than that. If you listen to your anger, figure out what fuels it and what needs to be changed in order to stop feeling negatively towards it, you can use the energy of the emotion to accomplish that change.
Being angry because you're offended by what someone said or did... it depends on the situation. You need to choose your battles, sometimes it's not worth it and you'll gain more from brushing it off and letting go of the emotion.
Look at it like this; Did what they said have any truth to it? Analyze it and see if you can get something constructive out of it. Was what they said just bullshit and will the persons most likely not listen to your arguments if you tried talking to them? If so, then their words had no value in the first place and can easily be brushed off, there's no need to care for bullshit.