Are there any legitimate uses for the idiom 'needless to say' when you're writing?

Answer 1

Strictly, since the use of this idiom in writing is contrary to the medium (writing), there is no legitimate use of "say."

Answer 2

Absolutely. You can start a sentence that way for emphasis to show confidence that most people agree with what you're about to write.

Answer 3

needless to say, they found that the color of the sky is actually blue

Answer 4

Needless to say I don't need to say that.