I breed for temperment. I wouldn't breed from a very nervous or viscious hamster, they have to be tame and friendly and people orientated.
I have bought a few hamsters from breeders and they have all had very nice temperments. The only hamster I have had which bit me consistently for over a year came from
[email protected], other than that I have had one nip from a baby hamster the second day I had him which I felt was excusable.
Also, with hamsters, getting them used to being handled from an early age does make a difference. It takes longer to tame a 6 week old hamster that has never been handled than a 14 day one.