As Tudorp said, it depends on what it depends on. Gentle stuff gets my thumb, generally. Rapid triplet stuff may get the index down, thumb down, index up routine, or a three-finger fan roll starting with an up stroke for really fast triplets. My generic all-purpose strum is with three fingers together so I can slip in the roll easily. It's nothing I ever thought about, really, it just sort of happened that way. Plus, it's great for chunking. The hand is sort of curled into a fist, then it opens up over the strings and gets dampened with the open palm, if that makes sense. For hard rockin' numbers, I hold my thumb and index like I'm holding a pick, and downstroke the index nail while the heel of my hand dampens the bridge slightly. Old habits die hard. Don't even get me started on whether it's "proper" to play over the 12 fret... :}