[Sakurazuka]

I suppose it'd also depend on what type of guitar you're playing, if you're in the arcade or using the ASC it'll be different from if you're using the little plastic magic guitars (the flimsy ones) I don't have a home system, but I've seen and used at a friend's. but at the arcade for faster reactions or heavier requirement songs, I have the thumb pressing against the back of the neck, the neck resting on the edge of my hand at the base ofmy pinky. giving the fingers a bigger arch over the buttons so it's able to press down better.
for a more comfortable hold I have the neck resting in my palm, thumb wrapping around the back and top, more relaxed feel, but reaction can be a bit slower from time to time