Some of my snakes will hiss when touched--a few of them repeatedly, as they are picked up. Some of them will only hiss occasionally, and some never do. As was said, I think they just issue a warning when they really aren't in the mood to be touched. I only have one aggressive female, and with her, hissing is the least of our worries, lol. So, hissing doesn't lead automatically to striking, so long as you are very gentle and go slowly--I try to respect my snakes' moods if possible, and if not, I make the handling as brief as possible, unless the snake is one that isn't often handled and needs more socialization.