Somebody gave a club member an old trainer with a McCoy 19 redhead on it, complete with a yellow Tornado nylon prop. When they tried to start it, the prop threw a blade before it ever got started, and my recollection is that it didn't even backfire. If you gotta run those old props, boil them, for sure.
Modern plastic props are generally nylon with glass fiber molded in. There are something like 60 different types of "nylon", so they're likely not all the same. I don't think anybody boils the 'glass reinforced props to soften them, but we do have some locals who dye them for various reasons, and that does involve boiling water, so it must not hurt anything. They still break, when you hook up the controls backwards. Seen that happen.
Some modern props are made of "vinyl", without the glass fiber. Bolly Clubman and (I think) Thunder Tiger Cyclones are among these. They still break, but break somewhat differently...a chunk off of the tip, vs. the entire blade breaking off at the hub. This opinion comes from a fairly small sampling, however. Nobody I know has tried to boil one of them, but it might work the same, or not.
Steve