A123's I think are fine to charge in the plane through the balance port. I agree with always balancing. Make sure you trust yourself and your charger. It's easy to select the wrong chemestry or charge rates and that's when bad things happen.

I'm careful about it but I sometimes even charge Lipos in the plane .. that's when you need to be extra careful. Never more than 1C and monitor it like a hawk. I've never had a lipo catch fire at any charge rate but others have. The thing I have had with a lipo is a hole poked into the foil .... BAD NEWS. When charging the gas comes out of the hole and it's incredibly flamable. Dont use them .. with a hole in them the cell goes dead pretty soon anyway.

I have had NiMh batteries explode. Even when set to the right charge rate. They did not catch fire but the explosion was powerful to blow the pack apart.