Handling as babies does seem to help. I have one that I bought as an adult, who I don't believe was handled very much previously that is very interactive (follows me around the room and begs for food) but flips out if you try to pick him up. I raised all my others from babies, and they are fine with being handled. The nippiest ones are those that tend to get super excited about food; they're fine once they realize I'm not food though.

Honestly male/female seems to make a bigger difference than species as far as temperament goes. Males are more active, and seem more prone to remain jumpy and easily startled. Females tend to be calmer and less energetic; they're the ones that will just sit on you and chill out. That's held true for every garter I've had.