really depends on the fish, my piraya darts around, it is actually more skittish than my caribes.
the only non-skittish piranha is a dead one, I mean it won't move at all, must be not skittish at all. lol j/k
IME with red bellies, the young juveniles and the older adults tend to be less skittish. I think the juveniles are extremely curious and don't know any better, and the older adults seem to get used to people being in their presence. Anything in between is very skittish.
Like everyone else says, it depends on the fish and the tank setup. I currently have two large reds (8" & 9.5") in a 75 gallon. They will take smelts right out of my hand. They are in a planted 75 gallon.