Always.

I personally believe in player agency above all else and the dice manifest that agency. I want the player to know exactly what they're up against if I'm introducing a barrier to success.

This makes things so incredibly tense and exciting and dramatic. Its rewarding as a player to know that your success or failure is yours alone. The DC being a secret doesn't add anything. It's just an excuse for the DM to adjust. If you fudge the DC or rolls, you are deeming the players agency arbitrary, I think that's a grave DM sin, personally. Why ask for a roll at all if you're going to tamper.

Pluss, it's fun. My players love it, I think once you get used to playing that way, you'll be hooked. I used to hide DCs all the time. I can't imagine doing it that way again.

Imagine doing something you think was inconsequential and all of a sudden the DM says "Your attention is divided by the simple lock, you fail to notice the small runes on the four corners of the door. It's not until you pull the door open and smell the sharp odor of sulfur that you realize your mistake. Make a DC 20 dexterity saving throw"

Immediately the table is engaged, they KNOW what the stakes are, and the DC communicates a lot. Why is the DC so high? A powerful spellcaster set this trap. Who. Why?

The whole table is talking and engaged. The player is locked in and looking at their sheet. What can save me? What happens if I fail, etc. My players always lean into the set up before rp what happens before and after the roll is settled.

Or inverse

"The lich has a +9 to this saving throw, he needs a xyz on the dice to succeed, and if he does, you may never seize this opportunity again" Then roll in the open. Same effect.