ORIGINAL: slope-soarer
...As far as I can tell.... the original purpose of boiling nylon props was to relieve stresses induced in them during manufacture. These stresses would normally be relieved by ageing of the propeller, but because you don't know how old the prop is when you buy it the trend was to boil it and thus ensure the stresses were relieved.
You are correct. The first nylon props I remember are the white Grish Tornado props introduced in the mid nineteen fifties. The first batch were almost unbreakable...the combat guys loved them. The next batch were brittle and broke easily. My guess is that the first ones sat long enough to relieve the stresses before the props were introduced. Someone found that boiling them would relieve the stresses; word spread rapidly.
One problem with the Tornado nylons was that the prop arc was nearly invisible so some guys injured themselves. When Top Flite introduced their Nylon series, they had red stripes at the tip so you could see the arc. Grish solved the problem by dyeing their props yellow.
I would suggest looking up each brand and type before boiling them. Some brands vary composition with a particular series.
I have no idea which should be boiled and which should not these days. I haven't boiled a prop in years.
Good luck.
George