People use it when they are happy, when they are mad, during customs and rituals, as a challenge to fight, dances, and for many other uses. It is good to use it when you are with family and friends, during the custom of giving the sua (fa'aaloalo), and other harmless times and places.
It's not good to use it when there are other Samoans around, particularly strangers, because it will be like a challenge. Especially if people are drinking. It is one of the fastest ways to get yourself in the hospital or worse: the morgue. If you don't believe me, try walking into a housing project where a lot of Samoans live and yell it. Worse yet, try going to a village in Samoa and do that, you won't get out alive because what you are announcing is that you are the baddest toa (warrior) and you call out everybody and anybody, and that you don't care about the village laws and chiefs. It's a macho thing. Hope that sheds some light on the word choo hoo.
You: CHEE HOO!
*all the men and women come out and beat ur face*
and while you are on the ground, a Samoan child kicks you in the ribs