Depends on the hiss really. You get to know after a time with each snake what different hisses, body postures, etc. mean for that particular snake. We have two adults for instance, a male and female ball python, that hiss every single time they are moved or disturbed in any way. They've always done this and with those two it means absolutely nothing....just them grumbling basically since neither has ever bitten. The female is a powerful hunter though and will come out of her tub to "help" you feed her if you aren't fast enough. (Robin knows which female this one is LOL)

We have other snakes that if they put out a certain low, serious hiss I'm going to fully respect that warning and get the heck out of their strike range as fast as possible or at least block their strike or redirect them with a mist of water or a tap of the spritzer bottle. Really just comes down to knowing each snake's reactions, their cues and trying to give them no real reason to feel defensive around you.