its never safe, even a brand new lithium Cr123A can "vent with flame" otherwise known as explode..
the
only thing you can do is reduce the risk of that happening.
1) Use Brand-name CR123As, made in the USA. Panasonics from fenix-store are a good international source.
2) Do not mix batteries of differnt brands.
3) Do not use batteries with different levels of charge left in them
ie: only use two batteries in the x5 that were from the same light, such as an Incan surefire 2x Cr123a, do NOT use one Cr123a from a single cell light, and another one from a differnt single cell light.
4) once the levels hit very low, its time to throw them away, dont run them COMPLETELY down.
Crenshaw