[Bruno]

If it's something the PCs will notice, no matter the result on the roll, I roll in the open; I find having my players catching my typos and brainos is very useful. Attacks and defenses are the simplest examples; enemies trying to climb or jump or whatever, you can see what happened.

I may not tell them WHAT I'm rolling, sometimes. For instance if I'm pulling random rumours or treasures or names off a table, I won't be showing tables, or why I've ended up rolling 6 times in a row. But since it's something the PCs are aware of (or are becoming aware of due to some action: "I look in the treasure chest. What do I find?" [1], "What ho, good woman. Prithy, what is thy name?") I roll it in the open. If I want to reroll a result I don't like or fudge it or whatever, it doesn't matter that I rolled in the open because it's a table lookup and the dice are ultimately arbitrarily assigned to the table items.

I sometimes roll openly for things like wandering monsters, and sometimes I don't. As the whim takes me and depending on if I want to communicate "stuff happening" to the players on a meta level. We play online, and sometimes instead of visibly rolling dice I just send the message "GM rolls some dice." They don't know why, don't know the result, don't even know how many dice...

[1] "A trap."