As a player I don't like it when GMs fudge (in our favor or against it), so I don't fudge as a GM. I think the best approach is all dice rolls out in the open no fudging. That way everyone knows they are at least getting a fair shake.
My players definitely prefer that I roll out in the open. And, I agree that there is some excitement that comes from seeing the DM, rolling for an NPC, pull off a nifty trick or fail miserably.
In my last game session, I hand an NPC that had been getting the better of the PCs the game session before. When we picked up last game, the players came to the table prepared and ready to get this guy. I was hoping to turn the NPC into a recurring villian, but I didn't fudge it. And, the players did a good job with their plan. I reward good plans and good thinking, so they ended up getting the guy.
Not without effort though. The NPC almost got away. He grabbed a vine and swung, Tarzan like, to another limb of these giant trees--where all the action was taking place. The task to make swing was a DC 12 Tumble check. The NPC had no ranks in tumble and only a +1 for his DEX (a 3.5 based game).
I rolled this out in the open---and got a 17!
It was a pretty cool moment as this NPC almost got away. Rolling the dice out in front of the group was exciting.
But, there's another side to rolling the dice out in front the boys. And, it's a side I don't like at all.
First off, rolling dice in front of players gives the players too much information that they shouldn't have. Take the toss I describe above. They knew the DC of the Tumble check, and even if I don't tell them (didn't need to with a 17 throw), the players aren't dumb and can narrow down the stats and bonuses of an NPC after seeing a couple of throws.
But, that's not the biggest thing that irks me about rolling out in the open. I don't want the players focussed on dice throws. I don't want them playing a game. I want them hanging on my every word of description watching the events unfold in their heads.
I don't want them to see a low attack number and automatically think, "OK, my guy isn't hit."
I want them to see me roll behind the screen, not knowing anything about the toss, then hear how the bugbear in front of them narrowly missed swinging at them with this big, iron spiked club.
It's just more dramatic.
So, I've got a quandary....to roll or not to roll out in the open.