I've got one now that hisses, my girl Leeloo. She did that from Day 1 (now we're on Day 17, but who's counting?) LOL
After her settle in period and first feed, I've been doing some short handling sessions with her, as I'm pretty sure she wasn't handled much before I got her. I'll open up her tub and remove her hide, and she'll start hissing. I just reach on in and get her.
I've noticed more recently that now she'll give a little half hearted hiss - like - "hiss....oh, it's you - it doesn't work - never mind"
However, I do respect their boundaries. I'll take her out for about 5 minutes every few days (not day after day) until she relaxes and then put her back. I don't know if her hissing is cutting down because she's learning that it's not stopping the big bad human from picking her up, or if she's starting to make associations that nothing bad has happened to her when I've picked her up. Who knows what goes on in those little snakey heads? LOL