That said they generally resort to it before biting. I've encountered rattlers (usually without even seeing them) on many occasions. Usually they don't even rattle, they just slither away. But if not, they rattle, then you back away slowly. You practically have to step on a rattlesnake before it bites you.
They're pretty polite that way.
Possible exceptions for a pregnant mother rattlesnake. I've heard they're extremely aggressive. Never run into one, don't plan on it. Rattlesnakes also don't lay eggs, they give birth.