I suppose that you know the trick already, but if not: here it is:
If you have round knitting needles that stubbornly curl so that the work is no more fun anymore, lay them in hot (not boiling) water.
The plastic thread straightens up and the knitting is fun again.
I have very different needles (from plastic, wood, metal) and therefore I lay them in the hot water only for a very short time and see then whether it was enough. If not I repeat the procedure until the plastic threads lay flat.
But this way nothing can get in pieces.
Happy knitting! ;-))