As a manager, people are only ever trying to persuade you to “Yes” to their request. If your initial reply is “Yes”, they leave the room happy.
If your initial answer is “No”, they immediately try to persuade you to say “Yes”.
No one ever tries to change your mind to “No” after saying “Yes”.
Of course, this leads to a bias towards “Yes” answers, unless you develop a reputation for being a ruthless naysayer, in which case you risk scaring off perfectly good ideas.
How about trying this rather. When you’re unsure of whether the answer is “Yes” or “No”, ask the requester to convince you why the answer should be “No.”
If they can, then they clearly understand he risks and pitfalls and you can probably give a “Yes” answer. If he can’t convince you “No” is the right answer, then “No” is the right answer.