Conventional wisdom in roleplaying games is that the GM should always roll behind the screen. This way you can conceal information from the players about how easy or hard a time the enemy is having at hitting them and so forth.
It also goes that by concealing the rolls, you can fudge them easier if you need.
Over time, I have generally come down on the no-fudging side of things. I am fine with GM fiat deciding a situation. If you don't want the party to die from a trap they happened to stumble into on their way back, that's fine.
But in that case, simply don't have the trap trigger or just declare a few points of damage. Enough to make a point, not enough to kill anybody.
Some will argue that by still rolling the dice, you preserve the feeling of uncertainty but I am almost willing to guarantee that your players very quickly figure out if they are likely to die or not.
A few years back, I adopted a practice of simply rolling all my dice where the players can see them. Occasionally if a die rolls too far away, I'll even let one of the players report what it rolled, rather than looking at it myself.
I find that it creates far more tension in the game as they'll watch that accursed D20 roll across the table, and they know whether they scraped by with a bit of luck or if the monster was just having bad dice that day. It all adds to the fun and enjoyment of the game in my opinion.
Most notably, it also means they know you didn't fudge anything. The trick is, you still have plenty of scope to adjust difficulty on the fly.
Encounter is going too easy? Add in a few reinforcements, give the orc leader a magic potion or just beef up the next fight a little.
Looks like the heroes are getting trounced a bit early? Do a morale check for the monsters or have a third party show up and interfere with the encounter. Good opportunity for more chaos or more adventure opportunities.
Suddenly a rampaging owl bear crashes into the rear ranks of the hobgoblins. The heroes take the opportunity to disengage from a losing battle but now they have to contend with the owlbear later.
Do you roll in plain view or in secret? Share your practices and experiences in the comments.