My impression (as it might be wrong) is that the words "I think" are contextually appropriate depending on what you're trying to accomplish. If you want to voice an opinion for consideration but not push for it, then "I think" demonstrates your openness. If you are trying to argue for something directly and expect opposition, then you should state your case as directly and strongly as possible. A third option is when you are expected to be a voice of authority (ie. you're a tech and someone just asked you what Linux is). Authorities are supposed to know, and people will think you're BSing if you say "I think..." what you really know. I personally would prefer you admit if you're unsure, but some people would rather you act confident than disclaim.