We pretty much train with fingers open, as well. Actually, I don't know that we ever really talked about what to do with the fingers much, but I'm sure we will now. We still deal with extension, but mostly on a mechanical level (an arm, etc.) Actually, the thumb thing came up when we were talking about Shihonage, my instructor was saying that he was taught to essentially stick your thumb in your center in that initial hand turn-in. I've never heard of it being used in other techniques, I'll bring that up!