It varies quite a bit how I roll.
Usually, I roll where ever is convenient to the physical act of rolling the dice. Sometimes, I
definitely want to roll behind the screen. I roll behind the screen if I want the ability to
fudge the die roll, or if I want to prevent the players from drawing conclusions based on the die face.
The latter of the two is actually much more common. If the PCs are trying to be deceptive or sneaky, I typically roll behind the screen. I don't want them to see a natural 18 or a natural 5 and draw conclusions that their Stealth or Persuasion roll must be a success or a failure if the players have no immediate means to garner feedback. It's not at all uncommon for me to want what I describe to the players to be what the PCs perceive, not what I rolled.
I do it like this because I hate it when I'm a player and I know what I shouldn't. I know that if I rolled a 16 on my Persuasion check and the DM rolls a natural 17 on the die for his opposed Insight, then I know my check failed even if the DM doesn't tell me that explicitly. My character doesn't know that, but I as a player do. That's a really frustrating position to be in because now I know metagame information that I never needed access to because the DM decided to tell me about it. If you're the kind of person who has a lot of the game's math stuck in your head, then you often just
know what the outcomes are by seeing the die rolls. And I can't
not do it; my brain just solves that equation when it has enough information to do so.
When you play and run the game a lot, you start to notice that you can make these evaluations all the time. Usually it doesn't matter, but in most campaigns where DMs always roll in the open it reveals about two or three outcomes that I shouldn't know
every session. I'll even purposefully sit further away from the DM just to reduce the probability that I see something I don't want to know.
In an effort to prevent my players from enduring that, I roll behind the screen.