Some ammonia in the water really hustles the gunk off, and doesn't seem to harm the plastic, but I'm not positive about that...
Over time (and we're talking years, here) a couple of my rarer whistles have had the plastic fipple crack. I've glued them and then put a small hose clamp over the bottom of the bit, and tightened it down...makes 'em look kind of like clarinets, if you don't look too close, and it does amuse people.
Also have found time and travel changes the base of the bore on my whistles so they are no longer entirely round, and this sometimes improves the sound. N.B. Do not use a whistle as part of a percussion ensemble involving a wine bottle - even though it sounds pretty good; it fills the whistle with little dings that prevent it from playing after that.
Blessings,
Barbara