The statement "The world was created 6k years ago" could be a scientific/objective truth. So by adding "I think" people won't feel like a fool by disagreeing with you (otherwise they might be unknowingly disagreeing with a fact). On the other hand a statement like "Redheads are hot" is obviously an opinion. Prefacing that with "I think redheads are hot" sounds unnecessarily insecure to me, but I suspect most people would add the "I think" qualifier anyway.
I don't think it makes sense that a statement like "tomatoes are gross" is more threatening than "I think tomatoes are gross", but it might very well be the case.
On a tangent, I find that when people say something that I know to be (objectively) wrong, I tend to correct them by prefacing my opinion with "I think...", or "I heard that...", or "I read somewhere that...", and I always end my disagreement with a (tonal) question mark. Even when I make a statement or correct somebody, ending with a higher pitch (question mark style) works wonders to avoid offense.