As long as you have control over one end, it's usually okay... but the second I notice them starting to squeeze or curl around, I'll usually tap their nose (makes them back up) or lift them off. I didn't act fast enough once with Mona, and seriously almost passed out!Just gotta be careful, and remain in control - which goes for dealing with any animal, really.
P.S. I've also noticed it helps to offer a "branch" before they choose your neck... I usually let Toby curl his tail around my finger, and that's enough to keep him feeling stable.