Were the slaps real? There was a particular slap in "Who Done It?" that never fails to make me laugh out loud and cringe at the same time. After the fellows are untied, Larry answers the phone, and after his "We were tied up!" line, Moe comes through with this slap that looks REAL--you can see a bit of saliva on Larry's lower lip as a result. So, yes, while Moe may have had to soften the blows occasionally, I don't think it's possible for anyone to execute slaps like that (and that fast) by spreading out their fingers. And even if you did, it doesn't soften it that much.
Interestingly, Larry gave conflicting comments on Moe's slaps. In the "Stoogephile" (I can't remember the exact title, but it's the 1982 Jeff Forester book), he states that Moe's hands were soft, the slaps weren't that hard, and that Moe had a beautiful touch, "He could've been a pickpocket!" But in Larry's interview (the last one), he says that it was real, which actually makes sense, considering that it was unlikely that there would be that much change from the stage shows when it came to slapstick.