In most cases it is bluff, even my most laid back bull would do this at first, interestingly enough this is the only species that has not tagged me yet.

Donīt let the hissing and rattling intimidate you if you do you will never be able to handle your bull. And if your snake sense fear you will certainly get tagged as well.
Great advice right there ^. And, in the post prior to it.

The move is likely to stress the snake. A stressed snake is probably going to be less tolerant to handling.

And, if you continually back off from the bluffs, you will teach the snake that aggressive behavior results in being left alone. Ending up with a snake that isn't going to be handled/handle-able.
We get tagged when we are either being dumb(I've been dumb a couple times and my snakes have still not taken it upon themselves to teach me better) or when we mis-read bluffs for actual fear response when they are going to stand their ground.