I almost always have a spare in my laptop bag that goes through X-ray. I use a wide rubber band to cover the contacts as it tends to stay in place better than the original shipping caps. I do keep it in a pocket by itself as well. Consumer LiON batteries should all have internal short circuit and temperature runaway protection so it's unlikely they would actually combust but on some batteries those safeties are sacrificial which could leave you with a brick it they tripped.

The only time I ever recall anything being said about batteries was in Frankfurt when I was asked if I had more then five batteries, I didn't and no one looked the verify.

Cheers