Well, I say err on the side of keeping things 'behind the curtain.'
I'm experimenting with it myself in Metal Joe and Windy (though we haven't made so many rolls in Windy yet). I guess the thing to think about is why are you revealing this roll or that roll.
For example, in combat, if you reveal attack rolls then you should expect the players to work out the Defense, etc. of their opponents. Kind of 'how much do you want them to metagame' I suppose. Sometimes its a good thing, and sometimes it's not. I think it would tend to make me underestimate tough opponents and overestimate not so tough ones.
I should say I usually like to see damage rolls (just to know how much smack I'm putting down

), but on the other hand you can get enough of that through description too. Sometimes it's even better that way.
Okay, so that's a bunch of random babbling. Summary, start out with everything secret, and try revealing bits and pieces of the mechanics here and there, and then judge what results.