Most of the fires I have heard about were upon charging overly discharged cells or over/improper charging any pack. Improper discharge can also cause problems. In the past two years, one member of our group has had one set of cells get really hot -- we destroyed it in a dirt bunker -- and one set bulge and get real spongy. It was similarly destroyed. Both incidents were upon charging. As for the risk of a problem, he probably has a total of about 3 packs to use during a flying season and is the only member of our club who uses them regularly.
He was using a charger "designed" for LiPo's at the time, but swithced to a different battery charger this Summer and has not had any problems since. We are still very cautious, though. I would not mess with LiPo cells. John
Edit: By mess with, I mean abuse. I started using LiPo's in a powered glider this Fall. They are great batteries. You just need to treat them right.