I don't think temperament can be determined all that well in babies... I would rather a baby act all pissy towards me, as that is its natural defense instinct and shows and should be expected from a healthy specimen. I'd actually worry if it didn't at least hiss at me when taken out of its hide; I'd have to be pretty sure that the reason was the individual's temperament and not the sign of some health problem.

I guess this might be more true in some reptile species than others. As far as handling goes, all I'm concerned about is whether it will bite me or not. Beyond that, I don't expect any reptile to be particularly cuddly or friendly towards me. The most I could hope for is that it becomes accustomed enough to my presence not to trigger the fight or flight response, but as long as it's not biting me, I'm happy. In some species, tameness may be the result of psychological subjugation resulting in stress. If your herp will be perfectly still for you, try to check its heart rate to see if it really is at ease, or is scared senseless but has learned that struggling is futile.

Anyway, I've found that ball pythons in general are pretty docile, so cherry picking one for its temperament wouldn't be of great concern to me. There are the exceptions of course, but they are just that: exceptions. Assuming you are dealing with someone reputable, in which case all the choices should be healthy (although it never hurts to double check), I would just pick a ball python by its looks.