Based on my past experience, most cannibalism happens right after the first meal of fruit flies (which is usually 2 days after hatching). Before that, they are skittish and most of them die due to starvation rather then cannibalism if i never (or forgot to) feed them.
Yeah, Yen's right- damn mantids!!! - You leave them together hoping it'll be natural selection n all and you go in one morning to find they've all died of starvation