Benjamin Olson
Hero
These days I'm only ever DMing online, and since I use real dice they are, by necessity, hidden. I used to roll in the open for some of my groups though. It depended on the vibe of the table.
There's nothing to indicate in any of the rules that anyone, player or DM, hide rolls.
DnD 5e DMG pg 235 "Rolling behind the screen lets you fudge the results if you want to If two critical hits in a row would kill a character, you could change the second critical hit into a normal hit, or even a miss, Dont distort die rolls too often, though, and don't let on that your doing it. Otherwise your players might think they dont face any real risks-- or worse, that you're playing favorites."
Page 237, 5E DMG "Remember that dice don't run your game - you do. Dice are like rules. They're tools to help keep the action moving."
If I weren't good with things being determined by die rolls (good/bd/otherwise) I wouldn't roll to begin with.