"Venture" in used herein the sense of "dare/be brave," rather than sense of "gamble/risk."
When you venture to say something, you are being bold enough to say it. Usually, it's an opinion -
"May I venture to say how beautiful you look tonight?"
- but not always. "I venture to say" would probably be an overly formal wording for telling your boss's boss that he has a piece of toilet paper stuck to his shoe, but that would be a fact that it might take a lot of courage to state.