Hussar
Legend
My experience seems to match many others in this thread.
I used to fudge dice rolls quite a bit, usually in favour of the players, but occasionally against them. Often it was to lower the amount of damage rolled to avoid killing a PC or a TPK.
However, several years ago I just decided to start rolling in the open and let the dice fall where they may. I've definitely enjoyed that change as it makes combat much more exciting for both the players and myself. They know that what I roll is what I roll. So if I get on a hot streak, they could be in trouble. Similarly, they know that I won't have my big bad "make" a save even though I rolled a 1.
This has led to a couple of anti-climactic boss fights due to me rolling poorly on some saving throws. However, I think this is offset by other encounters being surprisingly difficult due to me rolling really well.
The only time I roll in secret is if I'm rolling spot checks to see if they notice someone, or sense motive checks to see if they could catch someone out lying. In those cases they shouldn't know how well or poorly they or myself rolled. So I'll make those rolls behind my hand.
This is the boat I'm in too. I've come to realise that the dice often tell better stories at the end of the day than I might. If nothing else, it allows me, the DM, to be surprised by the outcome. And, for me, that's a good thing.