Fingertips for me. A carry over from playing fiddle and guitar. If you think about it, you've got three knuckle sharing the load as opposed to just one if you go flat. Of course, if you've got a serious two button stretch, there's littel choice but flat. I think for speed and acuracy, you have to bend the fingers and use the tips.
Steve

Finger tips seems to work best for speed and dexterity. But it isn't always possible in all circumstances.

-David